includes/rts: Drop trailing comma

This trailing comma snuck in in a recent patch. There is nothing wrong with the
comma; it's perfectly valid C99, yet nevertheless Mac OS X's dtrace utility
chokes on it with,

    dtrace: failed to compile script rts/RtsProbes.d:
            "includes/rts/EventLogFormat.h", line 245: syntax error near "}"
    make[1]: *** [rts/dist/build/RtsProbes.h] Error 1

(cherry picked from commit be514a69)