Thank you for your reply.
Sorry I forgot to add. I do have it in system.
/usr/include/x86_64-linux-gnu/bits/types/struct_timespec.h in system
None of me projects with default system include paths, complained when I tried add smaphore.h to it. So it seems to me that there is a problem with UE build system.
I try to create link to /usr/include/bits ā /usr/include/x86_64-linux-gnu/bits, but that breaks in other beautiful ways 
FLY Iām using 4.27.2 release.