bpf/selftests: test_select_reuseport_kern: Remove unused header

bpf/selftests: test_select_reuseport_kern: Remove unused header

test_select_reuseport_kern.c is currently including <stdlib.h>, but it
does not use any definition from there.

Remove stdlib.h inclusion from test_select_reuseport_kern.c

Signed-off-by: Alexis Lothoré (eBPF Foundation) <alexis.lothore@bootlin.com>
Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>
Link: https://patch.msgid.link/20250227-remove_wrong_header-v1-1-bc94eb4e2f73@bootlin.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>