Indicate that Python is required to compile Wireshark.

Remove some trailing white space at the same time.

(The ./configure options in INSTALL are significantly out of date--that will
be the subject of a separate commit.)

Change-Id: I3c03f90dab099e6d3386a235f35b691e7ff671be
Reviewed-on: https://code.wireshark.org/review/3563
Reviewed-by: Daniel Black <danielblack@fedoraproject.org>
Reviewed-by: Evan Huus <eapache@gmail.com>