net: socket: fix `sys/_timespec.h` not found error

When building `native_sim` with CPP & POSIX enabled, the
POSIX's `unistd.h` header causes a compilation error as
`sys/_timespec.h` doesn't exist in host gcc lib.

Remove the 'zephyr/posix' prefix, and rely on
`CMakeLists.txt`s to do the routing accordingly:
- zephyr/lib/CMakeLists.txt:L11
- zephyr/lib/posix/options/CMakeLists.txt:L8

Signed-off-by: Yong Cong Sin <ycsin@meta.com>