patch 8.2.1310: configure with Xcode 12 fails to check for tgetent

Problem:    Configure with Xcode 12 fails to check for tgetent.
Solution:   Declare tgetent(). (Ozaki Kiichi, closes #6558)
diff --git a/src/auto/configure b/src/auto/configure
index 9c9a719..41b29c2 100755
--- a/src/auto/configure
+++ b/src/auto/configure
@@ -12106,7 +12106,7 @@
 $as_echo_n "checking for tgetent()... " >&6; }
   cat confdefs.h - <<_ACEOF >conftest.$ac_ext
 /* end confdefs.h.  */
-
+int tgetent(char *, const char *);
 int
 main ()
 {