selftests: netfilter: fix a build error on openSUSE

This patch fixed the following build error on openSUSE Leap 15.3:

=======================================================================
 gcc     nf-queue.c -lmnl -o tools/testing/selftests/netfilter/nf-queue
 nf-queue.c:13:10: fatal error: libmnl/libmnl.h: No such file or directory
  #include <libmnl/libmnl.h>
           ^~~~~~~~~~~~~~~~~
 compilation terminated.
=======================================================================

It is because libmnl.h is put in the directory of
"/usr/include/libmnl/libmnl/" on openSUSE, not "/usr/include/libmnl/":

 > rpm -ql libmnl-devel
 /usr/include/libmnl
 /usr/include/libmnl/libmnl
 /usr/include/libmnl/libmnl/libmnl.h
 /usr/lib64/libmnl.so
 /usr/lib64/pkgconfig/libmnl.pc

Suggested-by: Kai Liu <kai.liu@suse.com>
Signed-off-by: Geliang Tang <geliang.tang@suse.com>
Reviewed-by: Shuah Khan <skhan@linuxfoundation.org>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>