deb_autobake - DPKG_GENSYMBOLS_CHECK_LEVEL=4 to autocheck MDEV-30482

Ensure that symbols aren't removed and are documented when added to
ensure consistency.

Same to enable when MariaDB/server#2780 is
merged up to the latest.

master.cfg

@@ -176,7 +176,7 @@ f_deb_autobake.addStep(downloadSourceTarball())
 f_deb_autobake.addStep(steps.ShellCommand(command=util.Interpolate("tar -xvzf /mnt/packages/%(prop:tarbuildnum)s_%(prop:mariadb_version)s.tar.gz --strip-components=1")))
 # build steps
 f_deb_autobake.addStep(steps.Compile(logfiles={'CMakeCache.txt': './builddir/CMakeCache.txt'}, command=["debian/autobake-deb.sh"],
-    env={'CCACHE_DIR':'/mnt/ccache', 'DEB_BUILD_OPTIONS':util.Interpolate('parallel=%(kw:jobs)s', jobs=util.Property('jobs', default='$(getconf _NPROCESSORS_ONLN)'))}, description="autobake-deb.sh"))
+                                     env={'CCACHE_DIR':'/mnt/ccache', 'DPKG_GENSYMBOLS_CHECK_LEVEL': 4, 'DEB_BUILD_OPTIONS':util.Interpolate('parallel=%(kw:jobs)s', jobs=util.Property('jobs', default='$(getconf _NPROCESSORS_ONLN)'))}, description="autobake-deb.sh"))
 # upload binaries
 f_deb_autobake.addStep(steps.SetPropertyFromCommand(command="find .. -maxdepth 1 -type f", extract_fn=ls2string))
 #f_deb_autobake.addStep(steps.MultipleFileUpload(workersrcs=util.Property('packages'),