Add deb-extra-override(5) to dpkg-dev

Its addition has been forgotten in
22049e15c9b844f3ebef480dfebc588fbd5a301e.

debian/dpkg-dev.install

@@ -21,6 +21,7 @@ usr/share/man/{*/*,*}/deb-control.5
 usr/share/man/{*/*,*}/deb-version.5
 usr/share/man/{*/*,*}/deb-substvars.5
 usr/share/man/{*/*,*}/deb-override.5
+usr/share/man/{*/*,*}/deb-extra-override.5
 usr/share/man/{*/*,*}/deb-shlibs.5
 usr/share/man/{*/*,*}/deb-symbols.5
 usr/share/man/{*/*,*}/deb-triggers.5