Build errors due to -Werror=format

..
[ 33%] Building C object CMakeFiles/lthread.dir/src/lthread_sched.c.o
/tmp/lthread/src/lthread_sched.c: In function '_lthread_sched_event':
/tmp/lthread/src/lthread_sched.c:300:13: error: format '%ld' expects argument of type 'long int', but argument 3 has type 'int64_t' [-Werror=format]
cc1: all warnings being treated as errors
..

Fixed by using proper macros from inttypes.h for all printf format identifiers.