RegionHelper.h: include <limits> directly.

Includes are transitively imported by <string> in String8.h + String16.h
but that include is being removed.

Test: pass
Change-Id: I378d533ee5445bc0286c7535581a491f55ca36c9
diff --git a/include/private/ui/RegionHelper.h b/include/private/ui/RegionHelper.h
index a22b2cb..a86c586 100644
--- a/include/private/ui/RegionHelper.h
+++ b/include/private/ui/RegionHelper.h
@@ -17,6 +17,7 @@
 #ifndef ANDROID_UI_PRIVATE_REGION_HELPER_H
 #define ANDROID_UI_PRIVATE_REGION_HELPER_H
 
+#include <limits>
 #include <stdint.h>
 #include <sys/types.h>