feat: Use subsecond timestamps for include file check

feat: Use subsecond timestamps for include file check

To avoid a race condition, ccache disables the direct mode if an include
file has a too new mtime or ctime. Previously this check used one second
resolution timestamps, which meant that a generated include file often
would disable direct mode hits for up to one second. Now ccache uses
timestamps with subsecond resolution (nanoseconds on Linux), so the
direct mode will in practice no longer have to be disabled for generated
include files.