Fix missing #includes in system/core.

Change-Id: I4736e8c4f7bb8bb03640d26d58243efaa67dc75c
diff --git a/toolbox/sendevent.c b/toolbox/sendevent.c
index 4b5a761..4d0ca17 100644
--- a/toolbox/sendevent.c
+++ b/toolbox/sendevent.c
@@ -6,6 +6,7 @@
 #include <stdlib.h>
 #include <string.h>
 #include <sys/ioctl.h>
+#include <unistd.h>
 
 int sendevent_main(int argc, char *argv[])
 {