gnutls: fix do_configure by depending on gtk-doc

gnutls: fix do_configure by depending on gtk-doc

building gnutls_2.8.5.bb fails in do_configure with these messages:

[..]
| automake: unknown warning category `cross'
| doc/Makefile.am:27: ENABLE_GTK_DOC does not appear in AM_CONDITIONAL
| gtk-doc.make:7: GTK_DOC_USE_LIBTOOL does not appear in AM_CONDITIONAL
| doc/reference/Makefile.am:135:   `gtk-doc.make' included from here
| gtk-doc.make:50: ENABLE_GTK_DOC does not appear in AM_CONDITIONAL
| doc/reference/Makefile.am:135:   `gtk-doc.make' included from here
| gtk-doc.make:173: ENABLE_GTK_DOC does not appear in AM_CONDITIONAL
| doc/reference/Makefile.am:135:   `gtk-doc.make' included from here
| doc/reference/Makefile.am:147: ENABLE_GTK_DOC does not appear in AM_CONDITIONAL
| autoreconf: automake failed with exit status: 1
| FATAL: autoreconf execution failed.
NOTE: Task failed: /home/chris/oe/micro-dev/work/i686-oe-linux/gnutls-2.8.5-r0/temp/log.do_configure.14113
ERROR: TaskFailed event exception, aborting
ERROR: Build of /home/chris/oe/openembedded.org/recipes/gnutls/gnutls_2.8.5.bb do_configure failed
ERROR: Task 652 (/home/chris/oe/openembedded.org/recipes/gnutls/gnutls_2.8.5.bb, do_configure) failed
NOTE: Tasks Summary: Attempted 865 tasks of which 0 didn't need to be rerun and 1 failed.
ERROR: '/home/chris/oe/openembedded.org/recipes/gnutls/gnutls_2.8.5.bb' failed

Depending on gtk-doc fixes this.

Signed-off-by: Antonio Ospite <ospite@studenti.unina.it>
Acked-by: Holger Hans Peter Freyther <zecke@selfish.org>
Signed-off-by: Stefan Schmidt <stefan@datenfreihafen.org>