add vendoring

vendor/github.com/mattn/go-sqlite3/sqlite3_opt_fts5.go

@@ -0,0 +1,14 @@
+// Copyright (C) 2019 Yasuhiro Matsumoto <mattn.jp@gmail.com>.
+//
+// Use of this source code is governed by an MIT-style
+// license that can be found in the LICENSE file.
+
+// +build sqlite_fts5 fts5
+
+package sqlite3
+
+/*
+#cgo CFLAGS: -DSQLITE_ENABLE_FTS5
+#cgo LDFLAGS: -lm
+*/
+import "C"