pthread.h needs clockid_t

actually it gets this from time.h if _POSIX_C_SOURCE or any other
feature test macros are defined, but it breaks if they're not.

include/pthread.h

@@ -5,6 +5,7 @@ extern "C" {
 #endif
 
 #define __NEED_time_t
+#define __NEED_clockid_t
 #define __NEED_struct_timespec
 #define __NEED_sigset_t
 #define __NEED_pthread_t