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