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

Define the __STDC_VERSION_SETJMP_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/D59135

include/setjmp.h

@@ -37,6 +37,8 @@
 #ifndef _SETJMP_H_
 #define _SETJMP_H_
 
+#define __STDC_VERSION_SETJMP_H__	202311L
+
 #include <sys/cdefs.h>
 
 /* The size of the jmp_buf is machine dependent: */