glsl: Initialize ubo_binding_mask flags to zero.

Missed in commit e63bb298. Caused sporadic test failures, like
incorrect-in-layout-qualifier-repeated-prim.geom.

Cc: "10.0" <mesa-stable@lists.freedesktop.org>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>

src/glsl/ast_type.cpp

@@ -118,6 +118,7 @@ ast_type_qualifier::merge_qualifier(YYLTYPE *loc,
    ubo_layout_mask.flags.q.shared = 1;
 
    ast_type_qualifier ubo_binding_mask;
+   ubo_binding_mask.flags.i = 0;
    ubo_binding_mask.flags.q.explicit_binding = 1;
    ubo_binding_mask.flags.q.explicit_offset = 1;