build: Move tap/configure.in -> tap/configure.ac

This silences the following warnings:

aclocal: warning: autoconf input should be named 'configure.ac', not
'configure.in'
automake: warning: autoconf input should be named 'configure.ac', not
'configure.in'
automake: warning: autoconf input should be named 'configure.ac', not
'configure.in'

which is pretty nice in and of itself.

Signed-off-by: Anton Lofgren <alofgren@op5.com>