Add feature test macro to declare kill()

Avoid the following warning on some systems:

> warning: implicit declaration of function 'kill'
> [-Wimplicit-function-declaration]

app/src/sys/unix/command.c

@@ -1,3 +1,5 @@
+#define _POSIX_SOURCE // for kill()
+
 #include "command.h"
 
 #include <errno.h>