Truth: Move export to runtime_deps

There is no good reason why depending on //lib/truth should free
downstream libraries from needing a direct dep on //lib/guava or
//lib/junit. Exporting the deps was probably just oversight, since the
distinction between exports and runtime_deps in WORKSPACE is not
obvious, and there are probably other bad examples.

Change-Id: I1530106d642167eaa25222cc95c3996478728e15

java/com/google/gerrit/truth/BUILD

@@ -4,6 +4,7 @@ java_library(
     srcs = glob(["**/*.java"]),
     visibility = ["//visibility:public"],
     deps = [
+        "//lib:guava",
         "//lib/truth",
     ],
 )