packaging: Forbid compilation if the symbols file is outdated

An outdated symbols file results in automatic versioned dependencies
in projects which use this library.

When these projects are built, shlibs:Depends cannot find a version
number for the set of symbols so it uses the full version of the
library package, including suffixes such as +virginia.

We end up with depends lib (>== 6.0.0+virginia) instead of
lib (>== 6.0.0).

debian/rules

@@ -2,6 +2,7 @@
 
 export DEB_LDFLAGS_MAINT_APPEND = -Wl,--as-needed
 export DEB_BUILD_MAINT_OPTIONS = hardening=+all
+export DPKG_GENSYMBOLS_CHECK_LEVEL = 2
 
 %:
 	dh $@ --with=gir