Enabling Developer Toolset 7 for Jenkins' lo_tb_master_linux_dbg

...aka "Tinderbox on Master for Linux-DbgUtil",
<https://ci.libreoffice.org/job/lo_tb_master_linux_dbg/>

Change-Id: Icf99063e7aaf620cb13dd6ba869f98660f2de82a
Reviewed-on: https://gerrit.libreoffice.org/63982
Tested-by: Jenkins
Reviewed-by: Stephan Bergmann <sbergman@redhat.com>

distro-configs/Jenkins/Linux_dev_master.conf

 --enable-dbgutil
 --enable-kde4
+CC=/opt/rh/devtoolset-7/root/usr/bin/gcc
+CXX=/opt/rh/devtoolset-7/root/usr/bin/g++