wattsup-libftdi: ftdi_usb_purge_buffers is deprecated

[connorimes]

On Travis OSX builds, we're now getting a deprecation warning:

[ 73%] Building C object wattsup/libftdi/CMakeFiles/energymon-wattsup-libftdi.dir/__/energymon-wattsup.c.o
[ 73%] Building C object wattsup/libftdi/CMakeFiles/energymon-wattsup-libftdi.dir/wattsup-driver-libftdi.c.o
/Users/travis/build/connorimes/energymon/wattsup/libftdi/wattsup-driver-libftdi.c:80:7: warning: 'ftdi_usb_purge_buffers' is deprecated [-Wdeprecated-declarations]
  if (ftdi_usb_purge_buffers(ctx->ctx)) {
      ^
/usr/local/Cellar/libftdi/1.5_2/include/libftdi1/ftdi.h:566:9: note: 'ftdi_usb_purge_buffers' has been explicitly marked deprecated here
    int DEPRECATED(ftdi_usb_purge_buffers(struct ftdi_context *ftdi));
        ^
/usr/local/Cellar/libftdi/1.5_2/include/libftdi1/ftdi.h:247:42: note: expanded from macro 'DEPRECATED'
#define DEPRECATED(func) __attribute__ ((deprecated)) func
                                         ^
1 warning generated.

This appears to be new since libftdi 1.5 (2020-07-07): https://www.intra2net.com/en/developer/libftdi/

We need to consider backward compatibility before making changes, esp. with versions found in Linux repositories.