When checking for floorl(), include math.h.

It's not being found with MSVC, but is apparently present; perhaps
math.h does something magic, e.g. defining it as a macro, so try using
check_symbol_exists with "math.h".

Change-Id: Idaecac641fb0b87d399807d8bf1fcb74e7251a4e
Reviewed-on: https://code.wireshark.org/review/5911
Reviewed-by: Guy Harris <guy@alum.mit.edu>