libsys: export pdopenpid(2)

Reviewed by:	markj
Tested by:	pho
Sponsored by:	The FreeBSD Foundation
MFC after:	1 week
Differential revision:	https://reviews.freebsd.org/D57124

lib/libsys/Symbol.sys.map

@@ -391,6 +391,7 @@ FBSD_1.8 {
 };
 
 FBSD_1.9 {
+	pdopenpid;
 	pdrfork;
 	pdrfork_thread;
 	renameat2;

sys/sys/procdesc.h

@@ -137,6 +137,7 @@ pid_t	 pdfork(int *, int);
 pid_t	 pdrfork(int *, int, int);
 int	 pdkill(int, int);
 int	 pdgetpid(int, pid_t *);
+int	 pdopenpid(pid_t, int);
 int	 pdwait(int, int *, int, struct __wrusage *, struct __siginfo *);
 pid_t	 pdrfork_thread(int *, int, int, void *, int (*)(void *), void *);
 __END_DECLS