python: add license declaration to BUILD

Change-Id: I38714eb966545727bab3c5b5122083ec5ac30ee9

python/BUILD

diff --git a/python/BUILD b/python/BUILD
index fc81845..209751f 100644
--- a/python/BUILD
+++ b/python/BUILD
@@ -19,6 +19,8 @@
     "perfetto_py_library",
 )
 
+licenses(["notice"])
+
 perfetto_py_binary(
     name = "trace_processor_py_example",
     srcs = ["example.py"],