win32: The opengl headers should install to mesa/GL subdir
The subdir `mesa/GL` used to avoid conflict between Windows GL headers and mesa GL headers. The GL headers resident in Windows SDK are: ``` gl.h glaux.h glcorearb.h glext.h glu.h glxext.h wgl.h wglext.h ``` Signed-off-by: Yonggang Luo <luoyonggang@gmail.com>