ril: Fix build with ISO C++11 compilers after ABI fix

Fix implicit signed-to-unsigned cast after restoring
the old ABI for android::os::Parcel::writeString16

Change-Id: I6732fe2e4f2b45d636bf59278ed98efa979c4393
Signed-off-by: Bernhard Rosenkraenzer <Bernhard.Rosenkranzer@linaro.org>
diff --git a/libril/ril.cpp b/libril/ril.cpp
index e1877a1..6d7f570 100644
--- a/libril/ril.cpp
+++ b/libril/ril.cpp
@@ -297,7 +297,7 @@
     char16_t *s16;
     size_t s16_len;
     s16 = strdup8to16(s, &s16_len);
-    p.writeString16(s16, s16_len);
+    p.writeString16((uint16_t*)s16, s16_len);
     free(s16);
 }