[FreeBSD]: fixed build on FreeBSD

I fixed it:

make  all-am
  CXX    busybee_mta.lo
  CXX    busybee_sta.lo
  CXX    busybee_st.lo
  CXX    busybee_mapper.lo
  CXX    busybee_returncode.lo
  CXX    busybee_single.lo
  CXX    busybee_utils.lo
In file included from busybee_utils.cc:31:
/usr/include/ifaddrs.h:34: error: 'u_int' does not name a type
*** [busybee_utils.lo] Error code 1