fix: fix missing syscall.h file issue on QNX8

[hnuduanyang]

Description

When compiling Fast DDS debug build on QNX 8, <sys/syscall.h> is not available, causing build failure. Added conditional include with fallback to compile successfully on QNX platforms.

Signed-off-by: Duan Yang Yang.Duan@zf.com

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• New feature has been added to the versions.md file (if applicable).
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[hnuduanyang]

hi,
@emiliocuestaf , @richiprosima This PR fixes the build error on QNX 8 when <sys/syscall.h> is missing. The fix adds a conditional include with fallback.
Please review when you have time, thanks!