slurmrestd - Remove errant cast in _write_fmt_header()

buffer can't be const as it is about to be xfree()ed.

Issue: 50189
diff --git a/src/slurmrestd/http.c b/src/slurmrestd/http.c
index 666dc3a..eeb35b3 100644
--- a/src/slurmrestd/http.c
+++ b/src/slurmrestd/http.c
@@ -456,7 +456,7 @@
 static int _write_fmt_header(conmgr_fd_t *con, const char *name,
 			     const char *value)
 {
-	const char *buffer = _fmt_header(name, value);
+	char *buffer = _fmt_header(name, value);
 	int rc = conmgr_queue_write_data(con, buffer, strlen(buffer));
 	xfree(buffer);
 	return rc;