tcpdump: add PACKAGECONFIG for smi

* libsmi is autodetected in configure, but in most cases disabled because of
  cross-compilation so keep it explicitly disabled
* resolves following difference in builds with and without libsmi built
  before tcpdump:
  4.7.4-r0-with/temp/log.do_configure:checking smi.h usability... yes
  4.7.4-r0-with/temp/log.do_configure:checking smi.h presence... yes
  4.7.4-r0-with/temp/log.do_configure:checking for smi.h... yes
  4.7.4-r0-with/temp/log.do_configure:checking for smiInit in -lsmi... yes
  4.7.4-r0-with/temp/log.do_configure:checking whether to enable libsmi... not when cross-compiling

  4.7.4-r0-without/temp/log.do_configure:checking smi.h usability... no
  4.7.4-r0-without/temp/log.do_configure:checking smi.h presence... no
  4.7.4-r0-without/temp/log.do_configure:checking for smi.h... no

Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>