st/mesa: fix struct vs. class compilation warning
st/mesa: fix struct vs. class compilation warning

glsl_to_tgsi_visitor is earlier defined as a class, not a struct.
Fixes MSVC warning.

NOTE: This is a candidate for the 8.0 branch.

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