Fixed redifinition warning (automake)

[BobuSumisu]

Basically what the error message tells one to do:

$ autoreconf --install
src/Makefile.am:12: warning: deprecated feature: target 'passivedns' overrides 'passivedns$(EXEEXT)'
src/Makefile.am:12: change your target to read 'passivedns$(EXEEXT)'
/usr/share/automake-1.14/am/program.am: target 'passivedns$(EXEEXT)' was defined here
src/Makefile.am:5:   while processing program 'passivedns'

Tested on Ubuntu 14.04 with GNU Autoconf 2.69 and GNU Automake 1.14.1

[thus]

Merged, thanks :)