patch 8.0.1735: flexible array member feature not supported by HP-UX

Problem:    Flexible array member feature not supported by HP-UX. (John
            Marriott)
Solution:   Do not use the flexible array member feature of C99.
diff --git a/src/structs.h b/src/structs.h
index e3ad867..35433b1 100644
--- a/src/structs.h
+++ b/src/structs.h
@@ -511,7 +511,7 @@
 struct buffblock
 {
     buffblock_T	*b_next;	/* pointer to next buffblock */
-    char_u	b_str[];	/* contents (flexible array) */
+    char_u	b_str[1];	/* contents (actually longer) */
 };
 
 /*
@@ -519,7 +519,7 @@
  */
 struct buffheader
 {
-    buffblock_T	*bh_first;	/* first block of the list */
+    buffblock_T	bh_first;	/* first (dummy) block of list */
     buffblock_T	*bh_curr;	/* buffblock for appending */
     int		bh_index;	/* index for reading */
     int		bh_space;	/* space in bh_curr for appending */