Fix compiler warning in media/mtp.

BUG=27152673

Change-Id: I0a5a7f2005bd76acf4d09353305b66af3fd29e4b
diff --git a/media/mtp/MtpPacket.cpp b/media/mtp/MtpPacket.cpp
index bab1335..35ecb4f 100644
--- a/media/mtp/MtpPacket.cpp
+++ b/media/mtp/MtpPacket.cpp
@@ -69,7 +69,7 @@
     char buffer[500];
     char* bufptr = buffer;
 
-    for (int i = 0; i < mPacketSize; i++) {
+    for (size_t i = 0; i < mPacketSize; i++) {
         sprintf(bufptr, "%02X ", mBuffer[i]);
         bufptr += strlen(bufptr);
         if (i % DUMP_BYTES_PER_ROW == (DUMP_BYTES_PER_ROW - 1)) {