g10: Fix make distcheck problem.

* g10/test.c: Include string.h.
(prepend_srcdir): New.  Taken from Libgcrypt.
(test_free): New.
* g10/t-keydb.c (do_test): Malloc the filename.
* g10/Makefile.am (AM_CPPFLAGS): Remove -DSOURCE_DIR
(EXTRA_DIST): Add t-keydb-keyring.kbx.

Using SOURCE_DIR should in general work but we have seen problems when
doing this in Libgcrypt. Using the srcdir variable gives us anyway
more flexibility and aligns with the way we do it in tests/openpgp.

• Signed-off-by: Werner Koch <wk@gnupg.org>