main/extensions: Don't advertise unknown extensions overrides with (-)

Previously setting:
MESA_EXTENSION_OVERRIDE=-GL_MESA_ham_sandwich

Would cause Mesa to advertise support for the GL_MESA_ham_sandwich
extension, even though the override specifically asked for it to be
disabled.

Signed-off-by: Jordan Justen <jordan.l.justen@intel.com>
Reviewed-by: Anuj Phogat <anuj.phogat@gmail.com>

src/mesa/main/extensions.c

@@ -555,7 +555,7 @@ get_extension_override( struct gl_context *ctx )
          break;
       }
       recognized = set_extension(ctx, ext, enable);
-      if (!recognized) {
+      if (!recognized && enable) {
          strcat(extra_exts, ext);
          strcat(extra_exts, " ");
       }