do not build fbsend with nofirebase

The nofirebase build tag should remove all build dependencies on
firebase.  The fbsend test tool depends on firebase (and is also only
useful in builds that use firebase).  Hence, disable it.

Signed-off-by: Antonio Enrico Russo <aerusso@aerusso.net>

tools/fbsend/main.go

@@ -1,3 +1,5 @@
+//go:build !nofirebase
+
 package main
 
 import (