Fixes clamdscan "Unable to determine the filepath given the file descriptor"

[finchy]

Fixes #1653

Updated the CheckFDPassing.cmake file to ensure proper support for file descriptor passing on OpenBSD by conditionally defining _XOPEN_SOURCE. The logic now attempts to compile without this definition first, and if that fails, retries with it. Additionally, included fdpassing.h at the beginning of clamdcom.c to ensure correct macro definitions.

[Mohataseem89]

This is a solid portability fix. i especially like that the CMake logic first attempts compilation without _XOPEN_SOURCE before redefining it conditionally that avoids unnecessarily altering feature test macros on platforms where it is not required

one question: did you verify whether redefining _XOPEN_SOURCE could implicitly affect other headers (e.g., socket or sys/stat behavior) on OpenBSD builds of clamdscan? Sometimes feature-test macros subtly change struct definitions