bpf, libbpf: fix quiet install_headers

Both btf.h and xsk.h headers are not installed quietly due to
missing '\' for the call to QUIET_INSTALL. Lets fix it.

Before:

  # make install_headers
    INSTALL  headers
  if [ ! -d '''/usr/local/include/bpf' ]; then install -d -m 755 '''/usr/local/include/bpf'; fi; install btf.h -m 644 '''/usr/local/include/bpf';
  if [ ! -d '''/usr/local/include/bpf' ]; then install -d -m 755 '''/usr/local/include/bpf'; fi; install xsk.h -m 644 '''/usr/local/include/bpf';
  # ls /usr/local/include/bpf/
  bpf.h  btf.h  libbpf.h  xsk.h

After:

  # make install_headers
    INSTALL  headers
  # ls /usr/local/include/bpf/
  bpf.h  btf.h  libbpf.h  xsk.h

Fixes: a493f5f9 ("libbpf: Install btf.h with libbpf")
Fixes: 379e2014 ("libbpf: add xsk.h to install_headers target")
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Andrii Nakryiko <andriin@fb.com>