selftests/bpf: Fix pkg-config call building sign-file

selftests/bpf: Fix pkg-config call building sign-file

When building sign-file, the call to get the CFLAGS for libcrypto is
missing white-space between `pkg-config` and `--cflags`:

  $(shell $(HOSTPKG_CONFIG)--cflags libcrypto 2> /dev/null)

Removing the redirection of stderr, we see:

  $ make -C tools/testing/selftests/bpf sign-file
  make: Entering directory '[...]/tools/testing/selftests/bpf'
  make: pkg-config--cflags: No such file or directory
    SIGN-FILE sign-file
  make: Leaving directory '[...]/tools/testing/selftests/bpf'

Add the missing space.

Fixes: fc97590668ae ("selftests/bpf: Add test for bpf_verify_pkcs7_signature() kfunc")
Signed-off-by: Jeremy Sowden <jeremy@azazel.net>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Reviewed-by: Roberto Sassu <roberto.sassu@huawei.com>
Link: https://lore.kernel.org/bpf/20230426215032.415792-1-jeremy@azazel.net
Signed-off-by: Alexei Starovoitov <ast@kernel.org>