glsl: Add missing bool case in glsl_type::get_scalar_type

glsl: Add missing bool case in glsl_type::get_scalar_type

Since the case was missing bec4->get_scalar_type() would return bvec4,
but vec4->get_scalar_type() would return float.

NOTE: This is a candidate for stable branches.

Signed-off-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Matt Turner <mattst88@gmail.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>