core: only generate backtraces if EXTRA_DEBUG is defined.

This fixes build on systems that have no backtrace API in the
libc (e.g. BSDs).

io_wait.h

@@ -94,7 +94,9 @@
 #ifdef __OS_linux
 #include <features.h> /* for GLIBC version testing */
 #endif
+#ifdef EXTRA_DEBUG
 #include "lib/dbg/backtrace.h"
+#endif
 
 #ifndef FD_TYPE_DEFINED
 typedef int fd_type;
@@ -732,13 +734,17 @@ inline static int io_watch_del(io_wait_h* h, int fd, int idx,
  if (!(idx>=0 && idx<h->fd_no)) {
  LM_CRIT("[%s] FD index check failed, idx=%d, max=%d"
  " operating on %d\n",h->name, idx, h->fd_no, fd );
+#ifdef EXTRA_DEBUG
  log_backtrace();
+#endif
  rla_dump();
  idx = -1;
  } else if (h->fd_array[idx].fd!=fd) {
  LM_CRIT("[%s] FD consistency check failed, idx=%d points to fd=%d,"
  " but operating on %d\n",h->name, idx, h->fd_array[idx].fd, fd );
+#ifdef EXTRA_DEBUG
  log_backtrace();
+#endif
  rla_dump();
  idx = -1;
  }