Fix missing #includes in system/core.

Change-Id: I4736e8c4f7bb8bb03640d26d58243efaa67dc75c
diff --git a/toolbox/r.c b/toolbox/r.c
index 6183677..b96cdb2 100644
--- a/toolbox/r.c
+++ b/toolbox/r.c
@@ -5,6 +5,7 @@
 #include <stdlib.h>
 #include <string.h>
 #include <sys/mman.h>
+#include <unistd.h>
 
 #if __LP64__
 #define strtoptr strtoull