libc: Add <time.h> C23 feature test macro

Define the __STDC_VERSION_TIME_H__ feature test macro as the
header fully conforms to C23.

Reviewed by:	fuz
Approved by:	fuz (mentor)
MFC after:	1 month
Differential Revision:	https://reviews.freebsd.org/D59134

include/time.h

@@ -34,12 +34,11 @@
  * SUCH DAMAGE.
  */
 
-/*
- */
-
 #ifndef _TIME_H_
 #define	_TIME_H_
 
+#define __STDC_VERSION_TIME_H__	202311L
+
 #include <sys/cdefs.h>
 #include <sys/_null.h>
 #include <sys/_types.h>