glsl: rename _restrict to restrict_flag

To fix MSVC compile breakage.  Evidently, _restrict is an MSVC keyword,
though the docs only mention __restrict (with two underscores).

Note: we may want to also rename _volatile to volatile_flag to be
consistent.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=74900
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>

src/glsl/ast.h

@@ -473,7 +473,7 @@ struct ast_type_qualifier {
 	 unsigned explicit_image_format:1;
 	 unsigned coherent:1;
 	 unsigned _volatile:1;
-	 unsigned _restrict:1;
+	 unsigned restrict_flag:1;
 	 unsigned read_only:1; /**< "readonly" qualifier. */
 	 unsigned write_only:1; /**< "writeonly" qualifier. */
 	 /** \} */