patch 8.0.1215: newer gcc warns for implicit fallthrough

Problem:    Newer gcc warns for implicit fallthrough.
Solution:   Consistently use a FALLTHROUGH comment. (Christian Brabandt)
diff --git a/src/buffer.c b/src/buffer.c
index 1bf692a..0d69b9d 100644
--- a/src/buffer.c
+++ b/src/buffer.c
@@ -4325,6 +4325,7 @@
 
 	case STL_OFFSET_X:
 	    base = 'X';
+	    /* FALLTHROUGH */
 	case STL_OFFSET:
 #ifdef FEAT_BYTEOFF
 	    l = ml_find_line_or_offset(wp->w_buffer, wp->w_cursor.lnum, NULL);
@@ -4336,6 +4337,7 @@
 
 	case STL_BYTEVAL_X:
 	    base = 'X';
+	    /* FALLTHROUGH */
 	case STL_BYTEVAL:
 	    num = byteval;
 	    if (num == NL)