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)