Fixes to include signal.h

POSIX defines signal.h. sys/signal.h should not be used as it is
c-library internal header which may or may not exist. Notably with
musl it generates warning of being incorrect.

Change-Id: Ia56b0aa1d84b5c590114867b1b384a624f39a6fc

apps/app_dahdiras.c

@@ -36,12 +36,7 @@ ASTERISK_REGISTER_FILE()
 
 #include <sys/ioctl.h>
 #include <sys/wait.h>
-#ifdef __linux__
-#include <sys/signal.h>
-#else
 #include <signal.h>
-#endif /* __linux__ */
-
 #include <fcntl.h>
 
 #include <dahdi/user.h>