m4: Const correctness for C23

On some platforms, e.g. Linux Clang 22.1.8 / glibc 2.43, strchr()
now implements the C23 behaviour where passing a const pointer to
strchr() also returns a const pointer.  This breaks m4 during the
bootstrap build, since it assumes the return value is always a
mutable pointer.

Since the returned value is never modified, simply make the
temporary const.

MFC after:	1 week
Reviewed by:	bapt, dim
Sponsored by:	The FreeBSD Foundation
Differential Revision:	https://reviews.freebsd.org/D58494

usr.bin/m4/misc.c

@@ -70,7 +70,7 @@ unsigned char *endpbb;			/* end of push-back buffer     */
 ptrdiff_t
 doindex(const char *s1, const char *s2)
 {
-	char *t;
+	const char *t;
 
 	t = strstr(s1, s2);
 	if (t == NULL)