mesa: move GL_ARB_debug_output code into new debug_output.c file

The errors.c file had grown quite large so split off this extension
code into its own file.  This involved making a handful of functions
non-static.

Acked-by: Timothy Arceri <timothy.arceri@collabora.com>

src/compiler/glsl/glsl_parser_extras.cpp

@@ -27,6 +27,7 @@
 
 #include "main/core.h" /* for struct gl_context */
 #include "main/context.h"
+#include "main/debug_output.h"
 #include "main/shaderobj.h"
 #include "util/u_atomic.h" /* for p_atomic_cmpxchg */
 #include "util/ralloc.h"