rpc: fix BUILD_TAG mispelling (BUILDTAG)

This ensures a manual or RPC update tries the right build tag,
rather than source, which is currently not setup

src/rpc/core_rpc_server.cpp

@@ -1491,8 +1491,8 @@ namespace cryptonote
   bool core_rpc_server::on_update(const COMMAND_RPC_UPDATE::request& req, COMMAND_RPC_UPDATE::response& res)
   {
     static const char software[] = "monero";
-#ifdef BUILDTAG
-    static const char buildtag[] = BOOST_PP_STRINGIZE(BUILDTAG);
+#ifdef BUILD_TAG
+    static const char buildtag[] = BOOST_PP_STRINGIZE(BUILD_TAG);
 #else
     static const char buildtag[] = "source";
 #endif