selftests/net: so_txtime: fix clang issues for target arch PowerPC

On powerpcle, int64_t maps to long long. Clang 9 threw:
warning: absolute value function 'labs' given an argument of type \
'long long' but has parameter of type 'long' which may cause \
truncation of value [-Wabsolute-value]
        if (labs(tstop - texpect) > cfg_variance_us)

Tested: make -C tools/testing/selftests TARGETS="net" run_tests

Fixes: af5136f9

 ("selftests/net: SO_TXTIME with ETF and FQ")
Signed-off-by: Tanner Love <tannerlove@google.com>
Acked-by: Willem de Bruijn <willemb@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>