patch 8.2.5061: C89 requires signal handlers to return void

Problem:    C89 requires signal handlers to return void.
Solution:   Drop RETSIGTYPE and hard-code a void return value.
diff --git a/src/pty.c b/src/pty.c
index 4c4e9e2..7cd01ae 100644
--- a/src/pty.c
+++ b/src/pty.c
@@ -188,7 +188,7 @@
 {
     int		f;
     char	*m;
-    RETSIGTYPE (*sigcld) SIGPROTOARG;
+    void (*sigcld) SIGPROTOARG;
     static char TtyName[32];  // used for opening a new pty-pair
 
     if ((f = posix_openpt(O_RDWR | O_NOCTTY | O_EXTRA)) == -1)
@@ -259,7 +259,7 @@
 {
     int f;
     char *name;
-    RETSIGTYPE (*sigcld) SIGPROTOARG;
+    void (*sigcld) SIGPROTOARG;
 
     /*
      * SIGCHLD set to SIG_DFL for _getpty() because it may fork() and
@@ -312,7 +312,7 @@
 {
     int		f;
     char	*m;
-    RETSIGTYPE (*sigcld) SIGPROTOARG;
+    void (*sigcld) SIGPROTOARG;
     // used for opening a new pty-pair:
     static char TtyName[32];