add clock.h for pc99 for consistent includes across platforms
add clock.h for pc99 for consistent includes across platforms

winksaville/libplatsupport,agacek/util_libs,agacek/libplatsupport,agacek/util_libs,seL4/libplatsupport,seL4/libplatsupport,agacek/libplatsupport,agacek/util_libs,agacek/util_libs,winksaville/libplatsupport