Add absl deps that stats headers use. (#237)

Previously the build picked these up transitively. It's better
to be explicit.

Author: g-easy

opencensus/stats/BUILD

@@ -34,6 +34,8 @@ cc_library(
     deps = [
         ":core",
         ":recording",
+        "@com_google_absl//absl/strings",
+        "@com_google_absl//absl/time",
     ],
 )