intel/decoder: mark total_length as MAYBE_UNUSED in gen_spec_load

intel/decoder: mark total_length as MAYBE_UNUSED in gen_spec_load

Only used, when asserts are enabled.

Fixes an unused-variable warning with GCC 8:
 ../../../src/intel/common/gen_decoder.c: In function 'gen_spec_load':
 ../../../src/intel/common/gen_decoder.c:535:47: warning: variable 'total_length' set but not used [-Wunused-but-set-variable]
     uint32_t text_offset = 0, text_length = 0, total_length;
                                                ^~~~~~~~~~~~

Signed-off-by: Kai Wasserbäch <kai@dev.carbon-project.org>
Reviewed-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>