Include <math.h> or <stdlib.h> as necessary to declare functions.

Various floating-point math functions require <math.h>, and abs()
requires <stdlib.h>.

Change-Id: I6831cfdb17eac3ce129b6800f0fe82fbcfef2d28
Reviewed-on: https://code.wireshark.org/review/6002
Reviewed-by: Guy Harris <guy@alum.mit.edu>