Fix missing #includes in system/core.

Change-Id: I4736e8c4f7bb8bb03640d26d58243efaa67dc75c
diff --git a/toolbox/smd.c b/toolbox/smd.c
index 91e495c..343dea7 100644
--- a/toolbox/smd.c
+++ b/toolbox/smd.c
@@ -3,6 +3,7 @@
 #include <string.h>
 #include <fcntl.h>
 #include <errno.h>
+#include <unistd.h>
 
 int smd_main(int argc, char **argv)
 {