intel/aubinator_error_decode: mark ret as MAYBE_UNUSED in main

intel/aubinator_error_decode: mark ret as MAYBE_UNUSED in main

Only used, when asserts are enabled.

Fixes an unused-but-set-variable warning with GCC 8:
 ../../../src/intel/tools/aubinator_error_decode.c: In function 'main':
 ../../../src/intel/tools/aubinator_error_decode.c:759:11: warning: variable 'ret' set but not used [-Wunused-but-set-variable]
        int ret;
            ^~~

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