core: Fix unused variable error in handle_show_sysinfo.

The previous fix broke the case
HAVE_SYSINFO = no
HAVE_SYSCTL = yes
HAVE_SWAPCTL = no
which occurs on FreeBSD 11.1 for example.

ASTERISK-26563

Change-Id: If77c39bc75f0b83a6c8a24ecb2fa69be8846160a