Allow toggle of MTP debug

Via command line in TWRP:
twrp set tw_mtp_debug 1

This will enable debugging for MTP. You will need to disable and
re-enable MTP in order to see the debug logging.

Change-Id: Ia7bb0c584e10a4322b65ecf80a67ed7ee836b38e
diff --git a/mtp/MtpDebug.h b/mtp/MtpDebug.h
index 12a23cc..b572b3b 100755
--- a/mtp/MtpDebug.h
+++ b/mtp/MtpDebug.h
@@ -43,6 +43,7 @@
 	static const char* getFormatCodeName(MtpObjectFormat code);
 	static const char* getObjectPropCodeName(MtpPropertyCode code);
 	static const char* getDevicePropCodeName(MtpPropertyCode code);
+	static void enableDebug();
 };