[OE-core] libglu: dependency on x11

[Gabriel Huau]

Hi,

I'm not sure to understand why do we have a dependency on X11 in the 
libGLU package? I might miss some pieces of information here, but IMO, 
it should only depends on opengl as the libGL.so could be generated 
without X11 or any Window system.

It actually breaks some builds where we depend on libGLU but we don't 
have X11.

Thanks,
Gabriel