netfilter: Fix build error

The commit <d30948ce> (netfilter: Changes to handle segmentation in
SIP ALG) breaks the build by defining the last argument of the function,
sip_tcp_skip_process (s16 *tdiff) incorrectly. Hence, fix the type by
changing it to a variable.

Change-Id: I5a336bd7aa47a649c24d7c34090f138b2e772bdc
Signed-off-by: Raghavendra Rao Ananta <rananta@codeaurora.org>