patch 8.0.1712: terminal scrollback is not limited

Problem:    Terminal scrollback is not limited.
Solution:   Add the 'terminalscroll' option.
diff --git a/src/option.h b/src/option.h
index d2ee545..45f1a36 100644
--- a/src/option.h
+++ b/src/option.h
@@ -849,6 +849,9 @@
 #ifdef FEAT_ARABIC
 EXTERN int	p_tbidi;	/* 'termbidi' */
 #endif
+#ifdef FEAT_TERMINAL
+EXTERN long	p_tlsl;		/* 'terminalscroll' */
+#endif
 #ifdef FEAT_MBYTE
 EXTERN char_u	*p_tenc;	/* 'termencoding' */
 #endif