Inject -Wno-error=format-overflow -Wno-error=format-truncation in debian/rules

In contrary to Debian, we use by default -Wformat-overflow=2
and -Wformat-truncation=2 in Apertis, but due to the use of -Werror this
package FTBFS with: "cc1: all warnings being treated as errors".
In order to avoid this error, we don't treat these warnings as errors.

Signed-off-by: Dylan Aïssi <dylan.aissi@collabora.com>