syscall: add SYS_CLOCK_SETTIME (266) for clock_settime impl

Add the SYS_CLOCK_SETTIME constant to the syscall number definitions.
The next number after SYS_CLOCK_GETTIME (265) is 266, which becomes
the kernel-side number for the new clock_settime syscall.

This is paired with a relibc platform/redox/mod.rs change that
replaces the clock_settime ENOSYS stub with a real sc::syscall2
implementation.

The kernel-side handler needs to be added in the Redox kernel
(repo at /tmp or via kernel source); the userspace side is now
ready.

src/number.rs

@@ -67,6 +67,7 @@ pub const SYS_SYNC: usize = SYS_CLASS_FILE | 119;
 pub const SYS_SYNCFS: usize = SYS_CLASS_FILE | 120;
 
 pub const SYS_CLOCK_GETTIME: usize = 265;
+pub const SYS_CLOCK_SETTIME: usize = 266;
 pub const SYS_FUTEX: usize = 240;
 pub const SYS_MPROTECT: usize = 125;
 pub const SYS_NANOSLEEP: usize = 162;