misc: Attempt to fix compiler warnings

'sighup_hdlr' defined but not used [-Wunused-function]
diff --git a/src/application.c b/src/application.c
index 79cc68a..1eee487 100644
--- a/src/application.c
+++ b/src/application.c
@@ -72,10 +72,12 @@
 
 struct log_target *osmo_stderr_target;
 
+#ifdef SIGHUP
 static void sighup_hdlr(int signal)
 {
 	log_targets_reopen();
 }
+#endif
 
 /*! \brief Ignore \ref SIGPIPE, \ref SIGALRM, \ref SIGHUP and \ref SIGIO */
 void osmo_init_ignore_signals(void)