gnutls: add `gtk-doc` as build dependency
gnutls: add `gtk-doc` as build dependency

Without it, `autoreconf -fiv` would fail thusly:

	autoreconf: running: gtkdocize --copy
	Can't exec "gtkdocize": No such file or directory at /usr/share/autoconf/Autom4te/FileUtils.pm line 293.

Signed-off-by: Johannes Schindelin <53fb8db7833fca1e1746dd8592f61048d350f64c@gmx.de>

dscho/MSYS2-packages,dscho/MSYS2-packages,git-for-windows/MSYS2-packages,git-for-windows/MSYS2-packages,git-for-windows/MSYS2-packages,git-for-windows/MSYS2-packages,dscho/MSYS2-packages,dscho/MSYS2-packages