Suppress unused variable warning in if_malo.c

With clang 15, the following -Werror warning is produced:

sys/dev/malo/if_malo.c:1573:8: error: variable 'ix' set but not used [-Werror,-Wunused-but-set-variable]
        u_int ix;
              ^

Here, 'ix' is a variable that is only used when MALO_DEBUG is defined.
Mark the variable as potentially unused, to suppress the warning.

MFC after: 3 days

(cherry picked from commit 218634014374de0032fcdd56656ed6b3650634d2)