libsys: export exterrctl symbol

Reviewed by:	brooks
Sponsored by:	The FreeBSD Foundation
MFC after:	2 weeks
Differential revision:	https://reviews.freebsd.org/D50483

Author: kostikbel

lib/libsys/Symbol.sys.map

@@ -378,6 +378,7 @@ FBSD_1.7 {
 };
 
 FBSD_1.8 {
+	exterrctl;
 	fchroot;
 	getrlimitusage;
 	kcmp;