added sys/signal.h inclusion for BSD compliance

dwm.c

@@ -27,6 +27,7 @@
 #include <stdlib.h>
 #include <string.h>
 #include <unistd.h>
+#include <sys/signal.h>
 #include <sys/types.h>
 #include <sys/wait.h>
 #include <X11/cursorfont.h>