lib{c,sys}: expose cap_sandboxed from libc/gen

It's a thin wrapper on cap_getmode() implemented in libc, not a system
call so the symbol should have been exposed by libc/gen/Symbol.map
alongside the implementation.

Reviewed by:	kib
Differential Revision:	https://reviews.freebsd.org/D44110

lib/libc/gen/Symbol.map

@@ -358,6 +358,7 @@ FBSD_1.2 {
 };
 
 FBSD_1.3 {
+	cap_sandboxed;
 	clock_getcpuclockid;
 	dirfd;
 	dup3;

lib/libsys/Symbol.sys.map

@@ -356,7 +356,6 @@ FBSD_1.3 {
 	cap_ioctls_limit;
 	__cap_rights_get;
 	cap_rights_limit;
-	cap_sandboxed;
 	chflagsat;
 	clock_getcpuclockid2;
 	connectat;