glsl: init packed in more constructors.

src/compiler/glsl_types.cpp:577: uninit_member: Non-static class member "packed" is not initialized in this constructor nor in any functions that it calls.

from Coverity.

Fixes: 659f333b (glsl: add packed for struct types)

Acked-by: Ilia Mirkin <imirkin@alum.mit.edu>
(cherry picked from commit b2d4d08a)