glsl: parse in/out types for interface blocks

glsl: parse in/out types for interface blocks

Previously only 'uniform' was allowed for uniform blocks.

Now, in/out can be parsed, but it will only be allowed for
GLSL >= 150.

Signed-off-by: Jordan Justen <jordan.l.justen@intel.com>
Reviewed-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>