hparams: add missing TensorFlow dependency

Summary:
Fixes an unintentional omission from #2130: `api.py` (not just its test)
actually does currently depend on TensorFlow.

Part of a reorganization to match the structure described here:
<#2139 (comment)>

wchargin-branch: hparams-api-refactor

tensorboard/plugins/hparams/BUILD

@@ -159,6 +159,7 @@ py_library(
     deps = [
         ":protos_all_py_pb2",
         ":summary",
+        "//tensorboard:expect_tensorflow_installed",
         "@org_pythonhosted_six",
     ],
 )