mesa: Include simple_list.h explicitly in errors.c.
mesa: Include simple_list.h explicitly in errors.c.

This seems to be the only user of simple_list in core mesa not
including the header explicitly.

Reviewed-by: Brian Paul <3cb4e1df5ec4da2c7c4af7c52cec8cf340a55a10@vmware.com>

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler