glsl: make _mesa_shader_stage_to_string() available to non-C++ code.
glsl: make _mesa_shader_stage_to_string() available to non-C++ code.

Reviewed-by: Brian Paul <brianp@vmware.com>

v2: Split from patch "mesa: Store gl_shader_stage enum in gl_shader objects."

Reviewed-by: Kenneth Graunke <bd2562f754ec92342f93f61c25d731e290a2ffa8@whitecape.org>

mapbox/glsl-optimizer,metora/MesaGLSLCompiler,djreep81/glsl-optimizer,metora/MesaGLSLCompiler,zeux/glsl-optimizer,wolf96/glsl-optimizer,tokyovigilante/glsl-optimizer,wolf96/glsl-optimizer,tokyovigilante/glsl-optimizer,mapbox/glsl-optimizer,dellis1972/glsl-optimizer,mapbox/glsl-optimizer,jbarczak/glsl-optimizer,bkaradzic/glsl-optimizer,zz85/glsl-optimizer,djreep81/glsl-optimizer,jbarczak/glsl-optimizer,benaadams/glsl-optimizer,zeux/glsl-optimizer,bkaradzic/glsl-optimizer,wolf96/glsl-optimizer,tokyovigilante/glsl-optimizer,benaadams/glsl-optimizer,bkaradzic/glsl-optimizer,mapbox/glsl-optimizer,benaadams/glsl-optimizer,metora/MesaGLSLCompiler,jbarczak/glsl-optimizer,jbarczak/glsl-optimizer,djreep81/glsl-optimizer,zz85/glsl-optimizer,djreep81/glsl-optimizer,benaadams/glsl-optimizer,bkaradzic/glsl-optimizer,zz85/glsl-optimizer,mapbox/glsl-optimizer,wolf96/glsl-optimizer,dellis1972/glsl-optimizer,zz85/glsl-optimizer,mcanthony/glsl-optimizer,zz85/glsl-optimizer,dellis1972/glsl-optimizer,benaadams/glsl-optimizer,jbarczak/glsl-optimizer,wolf96/glsl-optimizer,zeux/glsl-optimizer,zz85/glsl-optimizer,tokyovigilante/glsl-optimizer,zeux/glsl-optimizer,mcanthony/glsl-optimizer,djreep81/glsl-optimizer,dellis1972/glsl-optimizer,mcanthony/glsl-optimizer,mcanthony/glsl-optimizer,bkaradzic/glsl-optimizer,dellis1972/glsl-optimizer,zeux/glsl-optimizer,mcanthony/glsl-optimizer,tokyovigilante/glsl-optimizer,benaadams/glsl-optimizer