Suppress unused variable warning in tcp_stacks's rack.cHEADmain

With clang 15, the following -Werror warning is produced:

    sys/netinet/tcp_stacks/rack.c:17405:12: error: variable 'outstanding' set but not used [-Werror,-Wunused-but-set-variable]
                    uint32_t outstanding;
                             ^

The 'outstanding' variable was used later in the rack_output() function,
but refactoring in 35c7bb340788f removed the usage. To avoid too much
code churn, mark the variable unused to supress the warning.

MFC after:	3 days

0 files changed, 0 insertions, 0 deletions