glsl: fix new gcc6 warnings

glsl: fix new gcc6 warnings

src/compiler/glsl/ast_to_hir.cpp: In function ‘unsigned int ast_process_struct_or_iface_block_members(exec_list*, _mesa_glsl_parse_state*, exec_list*, glsl_struct_field**, bool, glsl_matrix_layout, bool, ir_variable_mode, ast_type_qualifier*,
unsigned int, unsigned int)’:
src/compiler/glsl/ast_to_hir.cpp:6339:52: warning: ‘first_member_has_explicit_location’ may be used uninitialized in this function [-Wmaybe-uninitialized]
             if (!layout->flags.q.explicit_location &&
                 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~
                 ((first_member_has_explicit_location &&
                 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
                   !qual->flags.q.explicit_location) ||
                   ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
                  (!first_member_has_explicit_location &&
                  ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
                   qual->flags.q.explicit_location))) {
                   ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Signed-off-by: Rob Clark <robdclark@gmail.com>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>