glsl: Have the lexer return LAYOUT_TOK if 420pack is enabled.

GL_ARB_shading_language_420pack also provides layout qualifiers.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Paul Berry <stereotype441@gmail.com>

src/glsl/glsl_lexer.ll

@@ -366,7 +366,8 @@ layout		{
 		      || yyextra->ARB_conservative_depth_enable
 		      || yyextra->ARB_explicit_attrib_location_enable
 		      || yyextra->ARB_uniform_buffer_object_enable
-		      || yyextra->ARB_fragment_coord_conventions_enable) {
+		      || yyextra->ARB_fragment_coord_conventions_enable
+                      || yyextra->ARB_shading_language_420pack_enable) {
 		      return LAYOUT_TOK;
 		   } else {
 		      yylval->identifier = strdup(yytext);