Tiny Makefile fix for --enable-sql

[chouser@gentoo.org]

In order to get either 1.6.7 or 1.6.8 to build when configured with
--enable-sql, I had to modify src/engine/Makefile.am.  The build was
failing with:

make[1]: *** No rule to make target `../libgncengine.la', needed by `libgnc_postgres.la'.  Stop.
make[1]: Leaving directory `/var/tmp/portage/gnucash-1.6.7/work/gnucash-1.6.7/src/engine/sql'

I was able to get the build to complete successfully by adding "." to
SUBDIRS in src/engine/Makefile.  This fix is now included in gentoo's
gnucash package: app-office/gnucash-1.6.8.ebuild.  If there is a more
appropriate way to fix this, please let me know.  Otherwise, I hope you
will consider including this fix in a future version of gnucash.

BTW, thanks for all your hard work on this great application!
--Chouser