Fix for AnalogIn.cpp:42: undefined references

Was getting the following errors compiling on in Cygwin on Win7 x32. Added empty functions to reflect header file to mimic setpin()

Full error msgs:
/cygdrive/c/MAVPRO~1/ardupilot/libraries/AP_HAL_Empty/AnalogIn.cpp:42: undefined reference to `Empty::EmptyAnalogSource::set_stop_pin(unsigned char)'
/cygdrive/c/MAVPRO~1/ardupilot/libraries/AP_HAL_Empty/AnalogIn.cpp:42: undefined reference to `Empty::EmptyAnalogSource::set_settle_time(unsigned short)'