fix libosmocore build for NuttX target

this fixes some compilation issues with libosmocore under NuttX,
particularly as some #defines are missing or some header files are
slightly different.
diff --git a/src/application.c b/src/application.c
index 881f011..9c3fe52 100644
--- a/src/application.c
+++ b/src/application.c
@@ -81,10 +81,16 @@
 void osmo_init_ignore_signals(void)
 {
 	/* Signals that by default would terminate */
+#ifdef SIGPIPE
 	signal(SIGPIPE, SIG_IGN);
+#endif
 	signal(SIGALRM, SIG_IGN);
+#ifdef SIGHUP
 	signal(SIGHUP, &sighup_hdlr);
+#endif
+#ifdef SIGIO
 	signal(SIGIO, SIG_IGN);
+#endif
 }
 
 /*! \brief Initialize the osmocom logging framework