Move license text to BUILD files top

PiperOrigin-RevId: 388579579

cpp/platform/impl/g3/BUILD

@@ -1,4 +1,3 @@
-licenses(["notice"])
 # Copyright 2020 Google LLC
 #
 # Licensed under the Apache License, Version 2.0 (the "License");
@@ -12,6 +11,7 @@ licenses(["notice"])
 # WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 # See the License for the specific language governing permissions and
 # limitations under the License.
+licenses(["notice"])
 
 cc_library(
     name = "types",