Fix compiler errors.

- Some platforms don't expose kill() unless _POSIX_C_SOURCE is defined.
- fork(), execl(), and setsid() need unistd.h on some platforms.

Basically, this fixes some platform-specific build errors.

sway/swaynag.c

@@ -1,3 +1,4 @@
+#define _POSIX_C_SOURCE 200809L
 #include <fcntl.h>
 #include <signal.h>
 #include <stdbool.h>