Let <sched.h> define struct timespec.

POSIX 2004 doesn't require that this header defines struct timespec, but
does allow it. For FreeBSD, we would at least need a forward declaration
for sched_rr_get_interval().

POSIX 2008 is a bit more demanding, as it now requires that it is
declared. Remove the old forward declaration and include the _timespec.h
header. This should conform to both POSIX 2004 and 2008 now.

sys/sys/sched.h

@@ -222,6 +222,7 @@ struct sched_param {
  */
 #ifndef _KERNEL
 #include <sys/cdefs.h>
+#include <sys/_timespec.h>
 #include <sys/_types.h>
 
 #ifndef _PID_T_DECLARED
@@ -229,8 +230,6 @@ typedef __pid_t         pid_t;
 #define _PID_T_DECLARED
 #endif
 
-struct timespec;
-
 __BEGIN_DECLS
 int     sched_get_priority_max(int);
 int     sched_get_priority_min(int);