contrib: Disable new dep gnutls in sysmocom SDK builds

Recently a new dependency on libgnutls was added to have a fallback for
kernels prior to 3.17 in which getrandom syscall fails, see libosmocore
ed029dfab959bca74bd43d86922727c2047eeb4d.

Since sysmobts has a kernel newer than 3.7 for a long time now, but we
are still running a 201310 image (and sdk) on them which doesn't contain
gnutls, let's disable it at configure time to fix the build against the
current SDK.

Change-Id: Iaa300c0eda2cf857f8c2915c6491efe583a12874
diff --git a/contrib/jenkins-build-osmo-bts-sysmo.sh b/contrib/jenkins-build-osmo-bts-sysmo.sh
index e636893..59ba85d 100755
--- a/contrib/jenkins-build-osmo-bts-sysmo.sh
+++ b/contrib/jenkins-build-osmo-bts-sysmo.sh
@@ -16,7 +16,7 @@
 # for gsm_data_shared.h
 have_repo openbsc
 
-build_repo libosmocore --disable-pcsc --disable-doxygen
+build_repo libosmocore --disable-pcsc --disable-doxygen --disable-gnutls
 build_repo libosmo-abis
 build_repo osmo-bts --enable-sysmocom-bts --with-openbsc=$base/openbsc/openbsc/include