patch 8.1.0759: showing two characters for tab is limited

Problem:    Showing two characters for tab is limited.
Solution:   Allow for a third character for "tab:" in 'listchars'. (Nathaniel
            Braun, Ken Takata, closes #3810)
diff --git a/src/globals.h b/src/globals.h
index 27b1562..4a0c52c 100644
--- a/src/globals.h
+++ b/src/globals.h
@@ -1163,6 +1163,7 @@
 EXTERN int	lcs_space INIT(= NUL);
 EXTERN int	lcs_tab1 INIT(= NUL);
 EXTERN int	lcs_tab2 INIT(= NUL);
+EXTERN int	lcs_tab3 INIT(= NUL);
 EXTERN int	lcs_trail INIT(= NUL);
 #ifdef FEAT_CONCEAL
 EXTERN int	lcs_conceal INIT(= ' ');