BUILD: quic: fix a build error about an non initialized timestamp

BUILD: quic: fix a build error about an non initialized timestamp

This is to please a non identified compilers which complains about an hypothetic
<time_ns> variable which would be not initialized even if this is the case only
when it is not used.

This build issue arrived with this commit:
	BUG/MINOR: improve BBR throughput on very fast links

Should be backported to 3.1 with this previous commit.

(cherry picked from commit 7868dc9c45aeb29774baac98711f563755b610d6)
Signed-off-by: Willy Tarreau <w@1wt.eu>