tools: Portability fix for gpgparsemail.

* tools/rfc822parse.c (my_stpcpy): Rename from stpcpy.

When HAVE_STPCPY is not defined (no definition in libc), still,
compiler may have definition as a built-in function (for some specific
case like second argument is constant). In that case, having stpcpy
implementation with the same name but different signature ("static"
qualifier) is not good.

• Signed-off-by: NIIBE Yutaka <gniibe@fsij.org>