DataFlash: narrow enumeration to uint8_t to avoid narrowing-warnings

/home/pbarker/rc/ardupilot/libraries/DataFlash/LogFile.cpp:1571:32:
error: non-constant-expression cannot be narrowed from type 'enum
LogMessages' to 'uint8_t' (aka 'unsigned char') in initializer list
[-Wc++11-narrowing]
        LOG_PACKET_HEADER_INIT(type),

libraries/DataFlash/LogStructure.h

@@ -1417,7 +1417,7 @@ Format characters in the format string for binary log messages
 // message types 0 to 63 reserved for vehicle specific use
 
 // message types for common messages
-enum LogMessages {
+enum LogMessages : uint8_t {
     LOG_NKF1_MSG = 64,
     LOG_NKF2_MSG,
     LOG_NKF3_MSG,