Fix missing #includes in system/core.

Change-Id: I4736e8c4f7bb8bb03640d26d58243efaa67dc75c
diff --git a/toolbox/touch.c b/toolbox/touch.c
index 52ddf2a..980f0d3 100644
--- a/toolbox/touch.c
+++ b/toolbox/touch.c
@@ -6,6 +6,7 @@
 #include <stdlib.h>
 #include <fcntl.h>
 #include <time.h>
+#include <unistd.h>
 
 static void usage(void)
 {