Don't show us the full Python command-line for building register.c.

The echo and output of make-dissector-reg.py is sufficient.

Change-Id: I89462435e8966012ec057ccd45adb595f692b0fb
Reviewed-on: https://code.wireshark.org/review/3587
Reviewed-by: Jeff Morriss <jeff.morriss.ws@gmail.com>