bpf: fix broken BPF selftest build on s390

With 720f228e ("bpf: fix broken BPF selftest build") the
inclusion of arch-specific header files changed.  Including the
asm/bpf_perf_event.h on s390, correctly includes the s390 specific
header file.  This header file tries then to include the s390
asm/ptrace.h and the build fails with:

cc -Wall -O2 -I../../../include/uapi -I../../../lib -I../../../../include/generated  -I../../../include    test_verifier.c
+/root/git/linux/tools/testing/selftests/bpf/libbpf.a /root/git/linux/tools/testing/selftests/bpf/cgroup_helpers.c -lcap -lelf -o
+/root/git/linux/tools/testing/selftests/bpf/test_verifier
In file included from ../../../include/uapi/asm/bpf_perf_event.h:4:0,
                 from ../../../include/uapi/linux/bpf_perf_event.h:11,
                 from test_verifier.c:29:
../../../include/uapi/../../arch/s390/include/uapi/asm/bpf_perf_event.h:7:9: error: unknown type name 'user_pt_regs'
 typedef user_pt_regs bpf_user_pt_regs_t;
         ^~~~~~~~~~~~
make: *** [../lib.mk:109: /root/git/linux/tools/testing/selftests/bpf/test_verifier] Error 1

This is caused by a recent update to the s390 asm/ptrace.h file
that is not (yet) available in the local installation.  That means,
the s390 asm/ptrace.h must be included from the tools/arch/s390
directory.

Because there is no proper framework to deal with asm specific
includes in tools/, slightly modify the s390 asm/bpf_perf_event.h
to include the local ptrace.h header file.

See also discussion on
https://marc.info/?l=linux-s390&m=151359424420691&w=2

Please note that this needs to be preserved until tools/ is able to
correctly handle asm specific headers.

References: https://marc.info/?l=linux-s390&m=151359424420691&w=2


Fixes: 720f228e ("bpf: fix broken BPF selftest build")
Signed-off-by: Hendrik Brueckner <brueckner@linux.vnet.ibm.com>
Cc: Daniel Borkmann <daniel@iogearbox.net>
Cc: Hendrik Brueckner <brueckner@linux.vnet.ibm.com>
Cc: Arnaldo Carvalho de Melo <acme@kernel.org>
Cc: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>