Fix compilation issues on OS X - mainly #include file changes (Lars Immisch)

diff --git a/src/signal.c b/src/signal.c
index a7fc28e..4227c6d 100644
--- a/src/signal.c
+++ b/src/signal.c
@@ -19,7 +19,7 @@
  */
 
 #include <openbsc/signal.h>
-#include <malloc.h>
+#include <stdlib.h>
 #include <string.h>