Fix build with Qt 5.6

A recent change [1] led to Qt5PlatformSupport.pc not being installed
anymore. Pkg-config was only used for finding the paths, and the actual
include flags and library names were hardcoded in KWin's
FindQt5PlatformSupport.cmake anyway, so let's just focus on finding
Qt5Gui, the module which provides this Qt5PlatformSupport library, and
work from there.

Thanks to Hrvoje Senjan (shumski) for pointing me towards the upstream
Gerrit change.

[1] https://codereview.qt-project.org/140954/

REVIEW: 126234

cmake/modules/FindQt5PlatformSupport.cmake

@@ -67,7 +67,7 @@ endif()
 # Use pkg-config to get the directories and then use these values
 # in the FIND_PATH() and FIND_LIBRARY() calls
 find_package(PkgConfig)
-pkg_check_modules(PKG_Qt5PlatformSupport QUIET Qt5PlatformSupport)
+pkg_check_modules(PKG_Qt5PlatformSupport QUIET Qt5Gui)
 
 set(Qt5PlatformSupport_DEFINITIONS ${PKG_Qt5PlatformSupport_CFLAGS_OTHER})
 set(Qt5PlatformSupport_VERSION ${PKG_Qt5PlatformSupport_VERSION})