Squelch a warning from MSVC.

Squelch a warning from MSVC.

Cast the result of a floating-point calculation to guint32; presumably
it will have no fractional part or we're deliberately discarding the
fractional part, and will never be greater than 2^32-1, so tell the
compiler we mean to discard the fractional part and have no need to
worry about the integral part being too large.

Change-Id: I432df46b59fe3192cd0a804d3d1db1f95983abe2
Reviewed-on: https://code.wireshark.org/review/28367
Reviewed-by: Guy Harris <guy@alum.mit.edu>