thread-utils.h: simplify the inclusion

thread-utils.h: simplify the inclusion

All files that include this header file use the same four line
incantation:

    #ifndef NO_PTHREADS
    #include <pthread.h>
    #include "thread-utils.h"
    #endif

Move the responsibility for that gymnastics to the header file from the
files that include it.  This approach makes it easier to later declare new
services that are related to threading in thread-utils.h and have them
available to all the threading code.

Signed-off-by: Junio C Hamano <gitster@pobox.com>