glsl: Less const for glsl_type convenience accessors

glsl: Less const for glsl_type convenience accessors

The second 'const' says that the pointer itself is constant.  This in
unenforcible in C++, so GCC emits a warning (see) below for each of
these functions in every file that includes glsl_types.h.  It's a lot of
warning spam.

../../../src/glsl/glsl_types.h:176:58: warning: type qualifiers ignored on function return type [-Wignored-qualifiers]

Signed-off-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Cc: mesa-stable@lists.freedesktop.org