patch 8.1.1229: warning for posix_openpt() not declared

Problem:    Warning for posix_openpt() not declared. (Tony Mechelynck)
Solution:   Add declaration.
diff --git a/src/pty.c b/src/pty.c
index ece360f..7a9e693 100644
--- a/src/pty.c
+++ b/src/pty.c
@@ -141,6 +141,7 @@
 char *ptsname(int);
 int unlockpt(int);
 int grantpt(int);
+int posix_openpt(int flags);
 #endif
 
     static void