neighbour: remove stray semicolon

Currently the stray semicolon means that the final term in the addition
is being missed.  Fix this by removing it. Cleans up clang warning:

net/core/neighbour.c:2821:9: warning: expression result unused [-Wunused-value]

Fixes: 82cbb5c6 ("neighbour: register rtnl doit handler")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Acked-By: Roopa Prabhu <roopa@cumulusnetworks.com>
Signed-off-by: David S. Miller <davem@davemloft.net>