libsa/netif.c: Replace #if with #ifdef

Follow the convention with *_DEBUG macros in stand/libsa/* and replace
"#if" with "#ifdef".

Reviewed by: kd
Obtained from: Semihalf
Differential Revision: https://reviews.freebsd.org/D36740

(cherry picked from commit 7b54d275ef6f74fe6030c85445bd3770d537d408)