selftests/bpf: Use syscall(SYS_gettid) instead of gettid() wrapper in bench

With glibc 2.28, selftests compilation fails for benchs/bench_trigger.c:

benchs/bench_trigger.c: In function ‘inc_counter’:
benchs/bench_trigger.c:25:23: error: implicit declaration of function ‘gettid’; did you mean ‘getgid’? [-Werror=implicit-function-declaration]
   25 |                 tid = gettid();
      |                       ^~~~~~
      |                       getgid
cc1: all warnings being treated as errors

It appears support for the gettid() wrapper is variable across glibc
versions, so may be safer to use syscall(SYS_gettid) instead.

Fixes: 520fad2e ("selftests/bpf: scale benchmark counting by using per-CPU counters")
Signed-off-by: Alan Maguire <alan.maguire@oracle.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20240322095728.95671-1-alan.maguire@oracle.com