dccp: re-enable debug macro

dccp tfrc: revert

This reverts 6aee49c558de ("dccp: make local variable static") since
the variable tfrc_debug is referenced by the tfrc_pr_debug(fmt, ...)
macro when TFRC debugging is enabled. If it is enabled, use of the
macro produces a compilation error.

Signed-off-by: Gerrit Renker <gerrit@erg.abdn.ac.uk>
Signed-off-by: David S. Miller <davem@davemloft.net>