Fix showing of swap details when sysinfo() is available

If sysinfo() is available, but not sysctl() or swapctl() the
printing code for swap buffer sizes is incorrectly omitted.
The above condition happens with musl c-library.

Fix #if rule to consider defined(HAVE_SYSINFO). And also
remove the redundant || defined(HAVE_SYSCTL) which was
incorrectly there to start with. Now swap information is
displayed only if an actual libc function to get it is
available.

This also fixes warnings previously seen with musl libc:

   [CC] asterisk.c -> asterisk.o
asterisk.c: In function 'handle_show_sysinfo':
asterisk.c:773:6: warning: variable 'totalswap' set but not used
 [-Wunused-but-set-variable]
  int totalswap = 0;
      ^~~~~~~~~
asterisk.c:770:11: warning: variable 'freeswap' set but not used
 [-Wunused-but-set-variable]
  uint64_t freeswap = 0;
           ^~~~~~~~

Change-Id: I1fb21dad8f27e416c60f138c6f2bff03fb626eca