qtbase: move virtual/libgl dependency to extra variable
qtbase: move virtual/libgl dependency to extra variable

* makes it easier to change desktop gl to gles2 in .bbappend

* we cannot explicitly disable opengl with -no-opengl, because then it
  fails to build (probably only when mesa headers are found in sysroot)
  /qtbase-opensource-src-5.0.0/src/platformsupport/eglconvenience/qeglplatformcontext.cpp:62:72: error: 'QPlatformOpenGLContext' has not been declared
  /qtbase-opensource-src-5.0.0/src/platformsupport/eglconvenience/qeglplatformcontext.cpp: In constructor 'QEGLPlatformContext::QEGLPlatformContext(const QSurfaceFormat&, int*, EGLDisplay, EGLenum)':
  /qtbase-opensource-src-5.0.0/src/platformsupport/eglconvenience/qeglplatformcontext.cpp:69:70: error: invalid static_cast from type 'int*' to type 'QEGLPlatformContext*'
  The same with 5.0.2

Signed-off-by: Simon Busch <e923d8431f09840b119134ca71ec8d74989d15cc@gravedo.de>
Signed-off-by: Martin Jansa <516df3ff67e1fa5d153800b15fb204c0a1a389dc@gmail.com>

Tofee/meta-qt5,kraj/meta-qt5,cjp256/meta-qt5,cjp256/meta-qt5,Ivan-SB/meta-qt5,sapiippo/meta-qt5,meta-qt5/meta-qt5,kraj/meta-qt5,Ivan-SB/meta-qt5,jruffin/meta-qt5,opendreambox/meta-qt5,timopulkkinen/meta-qt5,schnitzeltony/meta-qt5,meta-qt5/meta-qt5,timopulkkinen/meta-qt5,opendreambox/meta-qt5,schnitzeltony/meta-qt5,Tofee/meta-qt5,opendreambox/meta-qt5,jruffin/meta-qt5,timopulkkinen/meta-qt5,sapiippo/meta-qt5,timopulkkinen/meta-qt5