glsl: Don't emit empty declaration warning for a struct specifier

The intention is that things like

   int;

will generate a warning.  However, we were also accidentally emitting
the same warning for things like

  struct Foo { int x; };

Signed-off-by: Ian Romanick <ian.d.romanick@intel.com>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=68838
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Cc: Aras Pranckevicius <aras@unity3d.com>
Cc: "9.2 10.0" <mesa-stable@lists.freedesktop.org>

src/glsl/ast_to_hir.cpp

@@ -2940,7 +2940,7 @@ ast_declarator_list::hir(exec_list *instructions,
                                precision_names[this->type->qualifier.precision],
                                type_name);
          }
-      } else {
+      } else if (this->type->specifier->structure == NULL) {
          _mesa_glsl_warning(&loc, state, "empty declaration");
       }
    }