Let svg plugins extend svg module

svg plugins are by default not linked together with
QtGui since its usage is rare. But when the pro file
contains Qt += svg, then its the opposite; The
application is likely to need the svg plugins.

Many of the examples shipped with the SDK fails building
on iOS (e.g accelbubble) because of this.

Change-Id: I83873d5405fde2b3d38e836057d55fcc2ceb9b6c
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@digia.com>

src/plugins/iconengines/svgiconengine/svgiconengine.pro

 TARGET  = qsvgicon
 
 PLUGIN_TYPE = iconengines
-PLUGIN_EXTENDS = -
+PLUGIN_EXTENDS = svg
 PLUGIN_CLASS_NAME = QSvgIconPlugin
 load(qt_plugin)
 

src/plugins/imageformats/svg/svg.pro

 TARGET  = qsvg
 
 PLUGIN_TYPE = imageformats
-PLUGIN_EXTENDS = -
+PLUGIN_EXTENDS = svg
 PLUGIN_CLASS_NAME = QSvgPlugin
 load(qt_plugin)