glsl/glcpp: Swallow empty #pragma directives.

glsl/glcpp: Swallow empty #pragma directives.

Previously, we were passing these through, just like any other pragma. But the
downstream compiler was tripping up on them. It seems easier to swallow these
in the preprocessor and not pass them on at all rather than fixing the
downstream compiler.

This fixes the following Khronos GLES3 CTS tests:

	preprocessor.pragmas.pragma_vertex
	preprocessor.pragmas.pragma_fragment

Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>