glsl: Silence "ast_to_hir.cpp:1984:25: warning: comparison of unsigned expression >= 0 is always true"

ast_type_qualifier::location should have been a signed integer from
the beginning, and the giant comment in
apply_type_qualifier_to_variable explains why.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=40207
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>

src/glsl/ast.h

@@ -372,7 +372,7 @@ struct ast_type_qualifier {
     * \note
     * This field is only valid if \c explicit_location is set.
     */
-   unsigned location;
+   int location;
 
    /**
     * Return true if and only if an interpolation qualifier is present.