debian: Avoid -Wformat-zero-length warnings.

Debian puts an extra "-Wformat" in the CFLAGS following OVS's own
"-Wformat -Wno-format-zero-length", which therefore overrides
-Wno-format-zero-length, so this commit adds an extra
-Wno-format-zero-length to avoid those false positives.

Signed-off-by: Ben Pfaff <blp@nicira.com>
Acked-by: Justin Pettit <jpettit@nicira.com>