gbm: remove define _DEFAULT_SOURCE
gbm: remove define _DEFAULT_SOURCE

The build systems already add this as applicable. There's no need to
have this in the source file.

Signed-off-by: Emil Velikov <4c996f2f093ddd0585a556460b4cc3ba555546ef@gmail.com>

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler