patch 8.1.2021: some global functions can be local to the file

Problem:    Some global functions can be local to the file.
Solution:   Add "static". (Yegappan Lakshmanan, closes #4917)
diff --git a/src/pty.c b/src/pty.c
index 7a9e693..a2162e2 100644
--- a/src/pty.c
+++ b/src/pty.c
@@ -347,7 +347,7 @@
 #define PTY_DONE
 
 #ifdef _IBMR2
-int aixhack = -1;
+static int aixhack = -1;
 #endif
 
     int