DirectShow: Fix warnings of MinGW/gcc 5.3

helpers\directshowmediatype.h:70:58: note: in expansion of macro 'Q_DECL_NOTHROW'
     inline const AM_MEDIA_TYPE *const operator &() const Q_DECL_NOTHROW { return &mediaType; }
qtbase/src/corelib/global/qcompilerdetection.h:1143:26: warning: type qualifiers ignored on function return type [-Wignored-qualifiers]
 # define Q_DECL_NOEXCEPT noexcept                          ^
qtbase/src/corelib/global/qcompilerdetection.h:1153:25: note: in expansion of macro 'Q_DECL_NOEXCEPT'
 # define Q_DECL_NOTHROW Q_DECL_NOEXCEPT                         ^
helpers\directshowmediatype.h:71:59: note: in expansion of macro 'Q_DECL_NOTHROW'
     inline const AM_MEDIA_TYPE *const operator ->() const Q_DECL_NOTHROW { return &mediaType; }                                                           ^

camera\dscamerasession.cpp:604:24: warning: unused variable 'cLSID_SampleGrabber' [-Wunused-variable]
     static const CLSID cLSID_SampleGrabber = { 0xC1F400A0, 0x3F08, 0x11d3, { 0x9F, 0x0B, 0x00, 0x60, 0x08, 0x03, 0x9E, 0x37 } };
camera\dscamerasession.cpp:603:22: warning: unused variable 'iID_ISampleGrabber' [-Wunused-variable]
     static const IID iID_ISampleGrabber = { 0x6B652FFF, 0x11FE, 0x4fce, { 0x92, 0xAD, 0x02, 0x66, 0xB5, 0xD7, 0xC7, 0x8F } };
                        ^
helpers\directshowmediatype.cpp: In constructor 'DirectShowMediaType::DirectShowMediaType()':
helpers\directshowmediatype.cpp:79:57: warning: missing initializer for member '_AMMediaType::formattype' [-Wmissing-field-initializers]
     : mediaType({ GUID_NULL, GUID_NULL, TRUE, FALSE, 1 })
helpers\directshowmediatype.cpp:79:57: warning: missing initializer for member '_AMMediaType::pUnk' [-Wmissing-field-initializers]
helpers\directshowmediatype.cpp:79:57: warning: missing initializer for member '_AMMediaType::cbFormat' [-Wmissing-field-initializers]
helpers\directshowmediatype.cpp:79:57: warning: missing initializer for member '_AMMediaType::pbFormat' [-Wmissing-field-initializers]

camera\dscamerasession.cpp:805:111: warning: missing initializer for member '_AMMediaType::pUnk' [-Wmissing-field-initializers]
     static const AM_MEDIA_TYPE grabberFormat { MEDIATYPE_Video, MEDIASUBTYPE_RGB32, 0, 0, 0, FORMAT_VideoInfo };
camera\dscamerasession.cpp:805:111: warning: missing initializer for member '_AMMediaType::cbFormat' [-Wmissing-field-initializers]
camera\dscamerasession.cpp:805:111: warning: missing initializer for member '_AMMediaType::pbFormat' [-Wmissing-field-initializers]
camera\dscamerasession.cpp: In member function 'void DSCameraSession::presentFrame()':

camera\dscamerasession.cpp:585:52: warning: 'captureId' may be used uninitialized in this function [-Wmaybe-uninitialized]
         emit imageCaptured(captureId, captureImage);

Change-Id: I2870c1489bd375d3d43a31742551dab220015660
Reviewed-by: Yoann Lopes <yoann.lopes@qt.io>