Add missing include string.h

Include <string.h> for strdup() and strtok_r().

app/src/sys/unix/command.c

@@ -14,6 +14,7 @@
 #include <limits.h>
 #include <signal.h>
 #include <stdlib.h>
+#include <string.h>
 #include <sys/stat.h>
 #include <sys/types.h>
 #include <sys/stat.h>