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

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>