fix: use only PX4-specifc __PX4 symbol in time.c

[beniaminopozzan]

The current implementation of the of the logic to use PX4-specific time function:

#if defined(__PX4_NUTTX) || defined(__PX4_POSIX) || defined(UCLIENT_PLATFORM_NUTTX) || defined(UCLIENT_PLATFORM_POSIX)

ended up always selecting PX4 variation through the defined(UCLIENT_PLATFORM_NUTTX) || defined(UCLIENT_PLATFORM_POSIX) checks.

This was spotted in eProsima#408 and its check: https://github.com/eProsima/Micro-XRCE-DDS-Client/actions/runs/19801939470/job/56760212846

This PR proposed to change the check to

#if defined(__PX4)

and then let PX4 uxrce_dds_client module CMakeList.txt to add __PX4 to the submodule.

[beniaminopozzan]

@dagar , do you know of any better symbol (instead of __PX4)?

Initially I wanted to keep using __PX4_NUTTX and __PX4_POSIX but:

• The PX4-specific behaviour does not change based on nuttx or posix, so only one unified symbol is required
• Given the current way the this library is build in PX4: https://github.com/PX4/PX4-Autopilot/blob/main/src/modules/uxrce_dds_client/CMakeLists.txt I couldn't see __PX4_NUTTX or __PX4_POSIX being autimatically included. If I just reduce the checks to #if defined(__PX4_NUTTX) || defined(__PX4_POSIX) and recompile without altering the CMakeLists.txt, then the PX4 specific logic won't be built.

So I settled for __PX4 and added it to https://github.com/PX4/PX4-Autopilot/blob/main/src/modules/uxrce_dds_client/CMakeLists.txt in PX4/PX4-Autopilot#26045