nir/prog: Don't double-insert the fog-coord variable
nir/prog: Don't double-insert the fog-coord variable

nir_variable_create already inserts it in the right list for us so
inserting it again causes a linked list corruption.

Reviewed-by: Matt Turner <789b315743a28dd066a4ba1459c35951c291d8a6@gmail.com>

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler