Fix libsbuf sbuf_printf_drain symbol version

(Introduced incorrectly in r347229 earlier today.)

As pointed out by kevans, 1.6 should be used for FreeBSD 13, like r340383.

Submitted by:	kevans
Reported by:	kib
Reviewed by:	jilles
X-MFC-with:	 r347229
Differential Revision:	https://reviews.freebsd.org/D20187

lib/libsbuf/Symbol.map

@@ -37,6 +37,9 @@ FBSD_1.4 {
 
 FBSD_1.5 {
 	sbuf_putbuf;
-	sbuf_printf_drain;
 };
 
+/* Added in FreeBSD 13.x */
+FBSD_1.6 {
+	sbuf_printf_drain;
+};

lib/libsbuf/Version.def

@@ -11,3 +11,7 @@ FBSD_1.4 {
 
 FBSD_1.5 {
 } FBSD_1.4;
+
+# This version was first added to 13.0-current.
+FBSD_1.6 {
+} FBSD_1.5;