package/modem-manager: fix build without xsltproc

package/modem-manager: fix build without xsltproc

Commit 8efb52c1a17210dc79cfb61f56a71ffb760ccbe9 added a libxslt
dependency presumably to manage
https://github.com/freedesktop/ModemManager/commit/bbd39a457ce2949dd167f9c2e01eecf6a9e14716

However, this is wrong and build will fail on:

checking for xsltproc... no
configure: error: Please install xsltproc before configuring.

xsltproc is used to generate ModemManager-names.h since, at least,
version 0.7.990 and
https://github.com/freedesktop/ModemManager/commit/365b906a3e1bda05af10f92451a6b6ae60f8b879
However, this file is already available in the official tarball so drop
this unneeded dependency and set ac_cv_prog_XSLTPROC_CHECK to yes

Fixes:
 - http://autobuild.buildroot.org/results/edc755b874ea43d1c009ad76c28f05e18519138e

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>