Fix warning about QDeviceDiscovery violating ODR rule

When compiling with GCC 4.9's LTO, the compiler realizes that the class
looks different in two different compilation units and prints a warning.
Adding the necessary #define will make sure that the warning isn't
printed.

It's possible the warning indicates a real problem, if the class
actually got used in those two plugins. I wouldn't know.

QtPlatformSupport/private/.../qdevicediscovery_p.h:66:7: warning: type ‘struct QDeviceDiscovery’ violates one definition rule
.moc/.../qdevicediscovery_p.h:66:7: note: a type with the same name but different layout is defined in another translation unit

Change-Id: I73ca8e553e392b8d368f0deaa318d3e6635d73e1
Reviewed-by: Laszlo Agocs <laszlo.agocs@digia.com>

src/plugins/platforms/directfb/directfb.pro

@@ -29,6 +29,8 @@ HEADERS = qdirectfbintegration.h \
     qdirectfbscreen.h \
     qdirectfbeglhooks.h
 
+contains(QT_CONFIG, libudev): DEFINES += QDEVICEDISCOVERY_UDEV
+
 # ### port the GL context
 contains(QT_CONFIG, directfb_egl) {
     HEADERS += qdirectfb_egl.h

src/plugins/platforms/kms/kms.pro

@@ -9,6 +9,7 @@ QT += core-private gui-private platformsupport-private
 qtHaveModule(opengl):QT += opengl-private
 
 DEFINES += MESA_EGL_NO_X11_HEADERS __GBM__
+contains(QT_CONFIG, libudev): DEFINES += QDEVICEDISCOVERY_UDEV
 
 CONFIG += link_pkgconfig egl qpa/genericunixfontdatabase
 

src/plugins/platforms/linuxfb/linuxfb.pro

@@ -9,6 +9,7 @@ QT += core-private gui-private platformsupport-private
 
 SOURCES = main.cpp qlinuxfbintegration.cpp qlinuxfbscreen.cpp
 HEADERS = qlinuxfbintegration.h qlinuxfbscreen.h
+contains(QT_CONFIG, libudev): DEFINES += QDEVICEDISCOVERY_UDEV
 
 CONFIG += qpa/genericunixfontdatabase