Move code to timestamp_filter.cc

Change-Id: I714c3f064c6d2d4ad4ebf972246669ac3503a81f
diff --git a/aos/network/BUILD b/aos/network/BUILD
index c12c675..65dac75 100644
--- a/aos/network/BUILD
+++ b/aos/network/BUILD
@@ -357,6 +357,7 @@
 
 cc_library(
     name = "timestamp_filter",
+    srcs = ["timestamp_filter.cc"],
     hdrs = ["timestamp_filter.h"],
     deps = [
         "//aos/time",