Rename wsutil/floor.[ch] to wsutil/floorl.[ch].

That better indicates what they do - they don't supply floor(), as
that's a standard math.h feature dating back before C89, they supply
floorl().

Change-Id: Ib1278c51cdfc57680c28c51de87eafb2cb50c8eb
Reviewed-on: https://code.wireshark.org/review/5905
Reviewed-by: Guy Harris <guy@alum.mit.edu>