intel: aubinator: mark ftruncate_res as MAYBE_UNUSED in ensure_phys_mem

intel: aubinator: mark ftruncate_res as MAYBE_UNUSED in ensure_phys_mem

Only used, when asserts are enabled.

Fixes an unused-variable warning with GCC 8:
 ../../../src/intel/tools/aubinator.c: In function 'ensure_phys_mem':
 ../../../src/intel/tools/aubinator.c:209:11: warning: unused variable 'ftruncate_res' [-Wunused-variable]
        int ftruncate_res = ftruncate(mem_fd, mem_fd_len += 4096);
            ^~~~~~~~~~~~~

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