glsl: allow standalone semicolons outside main()

GLSL 4.60 offically added this but games and older CTS suites actually
had shaders that did this, we may as well enable it everywhere.

Adding stable because it appears apps in the wild do this.

Acked-by: Timothy Arceri <tarceri@itsqueeze.com>
Reviewed-by: Matt Turner <mattst88@gmail.com>
Cc: <mesa-stable@lists.freedesktop.org>
(cherry picked from commit babd1d526b)

src/compiler/glsl/glsl_parser.yy

@@ -2676,6 +2676,7 @@ external_declaration:
    | declaration            { $$ = $1; }
    | pragma_statement       { $$ = NULL; }
    | layout_defaults        { $$ = $1; }
+   | ';'                    { $$ = NULL; }
    ;
 
 function_definition: