Prevent stack corruption when using OpenGL Core Profile

Prevent stack corruption when using OpenGL Core Profile

When an OpenGL Core Profile context is requested the function
glGetStringi() is used to query the supported extensions as
glGetString(GL_EXTENSIONS) has been removed in the core profile.

The signature for glGetStringi used in Qt missed off the APIENTRY calling
convention. This results in stack corruption on windows each time
glGetStringi() is called leading to a crash.

Change-Id: Iff62c42e2bb5fc4a5c0561fae97ddc5a8ae3a45e
Reviewed-by: Samuel Rødal <samuel.rodal@nokia.com>