asterisk: add --without-libxslt to configure

asterisk: add --without-libxslt to configure

Since upstream commit b40c4d59b1dd803cad79060fb5b5a48d249ba578
"--disable-xmldoc" does no longer prevent the linking to libxslt, if
available. If that's the case one is greeted with the following error:

Package asterisk is missing dependencies for the following libraries:
libxslt.so.1

This commit explicitly disables the use of libxslt, to avoid the
dependency.

Signed-off-by: Sebastian Kemper <sebastian_ml@gmx.net>
(cherry picked from commit 64a1d2faef62352327c4aaeec803ce71ff5d2c40)