selftests/bpf: Copy test_kmods when installing selftest

Commit d6212d82 ("selftests/bpf: Consolidate kernel modules into
common directory") consolidated the Makefile of test_kmods. However,
since it removed test_kmods from TEST_GEN_PROGS_EXTENDED, the kernel
modules required by bpf selftests are now missing from kselftest_install
when "make install". Fix it by adding test_kmod to TEST_GEN_FILES.

Fixes: d6212d82 ("selftests/bpf: Consolidate kernel modules into common directory")
Signed-off-by: Amery Hung <ameryhung@gmail.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20250812175039.2323570-1-ameryhung@gmail.com