net: ipv4: typo issue, remove erroneous semicolon

Need remove erroneous semicolon, which is found by EXTRA_CFLAGS=-W,
the related commit number: c5441932
("GRE: Refactor GRE tunneling code")

Signed-off-by: Chen Gang <gang.chen@asianux.com>
Acked-by: Pravin B Shelar <pshelar@nicira.com>
Signed-off-by: David S. Miller <davem@davemloft.net>