posix: timer: build timer.c using the correct kconfig option

Previously timer.c was only built with CONFIG_POSIX_CLOCK=y even
though it has had its own Kconfig symbol (CONFIG_TIMER) for a
very long time.

Make POSIX_CLOCK imply TIMER rather than control whether it is
built, and adjust Kconfig / CMake as necessary.

Signed-off-by: Christopher Friedt <cfriedt@meta.com>