patch 8.1.2271: build error if FEAT_TAG_BINS is not defined

Problem:    Build error if FEAT_TAG_BINS is not defined. (John Marriott)
Solution:   Add #ifdef.

src/tag.c

@@ -2242,9 +2242,11 @@ parse_line:
 		lbuf = alloc(lbuf_size);
 		if (lbuf == NULL)
 		    goto findtag_end;
+#ifdef FEAT_TAG_BINS
 		// this will try the same thing again, make sure the offset is
 		// different
 		search_info.curr_offset = 0;
+#endif
 		continue;
 	    }