Suppress unused variable warning in tcp_stacks's rack.c

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

(cherry picked from commit 57cdd13d072e56bf68c7b3d3b446aaf01288e12b)