automake: Remove empty file variable.
automake: Remove empty file variable.

Fixes SCons build regression introduced with commit
a665cf1226b80ec52a0c1a4a38378df4389e8ebf.

Signed-off-by: Vinson Lee <25a259f709b0f1b620605d2518271a846d8826e5@freedesktop.org>
Tested-by: Vinson Lee <25a259f709b0f1b620605d2518271a846d8826e5@freedesktop.org>

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