Don't rely on -lrt from the toolchain

Change-Id: I17bdcb7f7be74c0dbdbb3168bcf8882045c87dcc
diff --git a/aos/ipc_lib/BUILD b/aos/ipc_lib/BUILD
index fed9a8a..5f896d6 100644
--- a/aos/ipc_lib/BUILD
+++ b/aos/ipc_lib/BUILD
@@ -91,6 +91,7 @@
     srcs = [
         "ipc_comparison.cc",
     ],
+    linkopts = ["-lrt"],
     deps = [
         "//aos:condition",
         "//aos:event",