Disable -Werror

It's very rare to want -Werror in distribution packages: it elevates
minor build warnings (in this case -Wmissing-field-initializers,
which doesn't really even indicate a problem) to build failures.
Signed-off-by: Simon McVittie <smcv@collabora.com>

debian/rules

@@ -2,3 +2,8 @@
 
 %:
 	dh $@ --buildsystem=cmake
+
+override_dh_auto_configure:
+	dh_auto_configure -- \
+		-DTREAT_WARNINGS_AS_ERRORS=OFF \
+		$(NULL)