Fix for ettercap 0.8.1 lua cmake error on osx 10.10

This is a small patch for fixing lua cmake error on OS X 10.10 Yosemite.

like this :

CMake Error at src/CMakeLists.txt:191 (SET_TARGET_PROPERTIES):
  set_target_properties Can not find target to add properties to: ettercap

http://pastebin.com/WS9wwuKj