bpf: Test_progs, fix test_get_stack_rawtp_err.c build

The linux-5.4.y commit 8781011a ("bpf: Test_progs, add test to catch
retval refine error handling") fails to build when libbpf headers are
not installed, as it tries to include <bpf/bpf_helpers.h>:

  progs/test_get_stack_rawtp_err.c:4:10:
      fatal error: 'bpf/bpf_helpers.h' file not found

For 5.4-stable (only) the new test prog needs to include "bpf_helpers.h"
instead (like all the rest of progs/*.c do) because 5.4-stable does not
carry commit e01a75c1

 ("libbpf: Move bpf_{helpers, helper_defs,
endian, tracing}.h into libbpf").

Signed-off-by: Kamal Mostafa <kamal@canonical.com>
Fixes: 8781011a

 ("bpf: Test_progs, add test to catch retval refine error handling")
Cc: <stable@vger.kernel.org> # v5.4
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>