Frameworks/base: Fix more warnings

Exposed by switching the target to Clang, and GCC 4.9 used by
MIPS.

Change-Id: Icb79285ab2306c39c2d381e53ea2e643ee2d2947
diff --git a/libs/androidfw/BackupHelpers.cpp b/libs/androidfw/BackupHelpers.cpp
index 36a4cfe..c78593d 100644
--- a/libs/androidfw/BackupHelpers.cpp
+++ b/libs/androidfw/BackupHelpers.cpp
@@ -555,7 +555,7 @@
     snprintf(buf + 124, 12, "%011llo", (isdir) ? 0LL : s.st_size);
 
     // [ 136 :  12 ] last mod time as a UTC time_t
-    snprintf(buf + 136, 12, "%0lo", s.st_mtime);
+    snprintf(buf + 136, 12, "%0lo", (unsigned long)s.st_mtime);
 
     // [ 156 :   1 ] link/file type
     uint8_t type;