runtime: fix type descriptor name in C code

runtime: fix type descriptor name in C code

    I forgot to update the name of the map[string]bool type descriptor
    used in go-fieldtrack.c.  This didn't cause any errors because it's a
    weak symbol, and the current testsuite has no field tracking tests.

    Reviewed-on: https://go-review.googlesource.com/91096

From-SVN: r257249