Fix MNG imageformat plugin build.

Trying to build MNG image format plugin for WEC7 gives the error:
Cannot open include file: 'qmnghandler_p.h': No such file or directory

The correct INCLUDEPATH was already used in qmnghandler.pri, but it
wasn't included anywhere. Refactored mng.pro to use qmnghandler.pri
correspondingly as TIFF image format plugin uses qtiffhandler.pri

Change-Id: Ie96c898b27747da3a34465eda2c6a3b923ad22c6
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@digia.com>
Reviewed-by: Johannes Oikarinen <johannes.oikarinen@digia.com>
Reviewed-by: Andreas Holzammer <andreas.holzammer@kdab.com>
Reviewed-by: aavit <eirik.aavitsland@digia.com>

src/plugins/imageformats/mng/mng.pro

@@ -4,15 +4,6 @@ PLUGIN_TYPE = imageformats
 PLUGIN_CLASS_NAME = QMngPlugin
 load(qt_plugin)
 
-HEADERS += qmnghandler_p.h
-SOURCES += qmnghandler.cpp
-OTHER_FILES += mng.json
-config_libmng {
-    unix|win32-g++*: LIBS += -lmng
-    else:win32: LIBS += libmng.lib
-} else {
-    include($$PWD/../../../3rdparty/libmng.pri)
-    *-g++*: QMAKE_CFLAGS_WARN_ON += -Wno-unused-parameter
-}
-
+include(qmnghandler.pri)
 SOURCES += main.cpp
+OTHER_FILES += mng.json

src/plugins/imageformats/mng/qmnghandler.pri

@@ -2,9 +2,10 @@
 INCLUDEPATH *= $$PWD
 HEADERS += $$PWD/qmnghandler_p.h
 SOURCES += $$PWD/qmnghandler.cpp
-contains(QT_CONFIG, system-mng) {
-        if(unix|win32-g++*):LIBS += -lmng
-        else:win32:         LIBS += libmng.lib
+config_libmng {
+    unix|win32-g++*: LIBS += -lmng
+    else:win32: LIBS += libmng.lib
 } else {
-    include($$PWD/../../3rdparty/libmng.pri)
+    include($$PWD/../../../3rdparty/libmng.pri)
+    *-g++*: QMAKE_CFLAGS_WARN_ON += -Wno-unused-parameter
 }