init: clean up more headers

We don't need everyone including <sys/system_properties.h>

Test: boot bullhead
Change-Id: I73d507e4f273678eaf15947725741e1e3b966cc6
diff --git a/init/builtins.cpp b/init/builtins.cpp
index e0a98f5..7298dff 100644
--- a/init/builtins.cpp
+++ b/init/builtins.cpp
@@ -33,6 +33,7 @@
 #include <sys/socket.h>
 #include <sys/stat.h>
 #include <sys/syscall.h>
+#include <sys/system_properties.h>
 #include <sys/time.h>
 #include <sys/types.h>
 #include <sys/wait.h>