Link advapi32.lib when linking crypto.

This is needed for RtlGenRandom [0] in crypto/rand_extra/windows.c [1].

Linker error actually shows "SystemFunction036" instead of "RtlGenRandom".

[0]: https://msdn.microsoft.com/en-us/library/windows/desktop/aa387694(v=vs.85).aspx
[1]: https://github.com/google/boringssl/blob/f21650709a6f76e829ddcc77fe221c9d6a5c12de/crypto/rand_extra/windows.c#L44

Change-Id: Ie52e0a1d507e3598ba3ae3c449d0088981c06d8a
Reviewed-on: https://boringssl-review.googlesource.com/28884
Reviewed-by: Adam Langley <agl@google.com>
Reviewed-by: David Benjamin <davidben@google.com>
Commit-Queue: Adam Langley <agl@google.com>
CQ-Verified: CQ bot account: commit-bot@chromium.org <commit-bot@chromium.org>
diff --git a/util/BUILD.toplevel b/util/BUILD.toplevel
index 6e569ec..6691f71 100644
--- a/util/BUILD.toplevel
+++ b/util/BUILD.toplevel
@@ -128,6 +128,7 @@
         # Android supports pthreads, but does not provide a libpthread
         # to link against.
         ":android": [],
+        ":windows_x86_64": ["-defaultlib:advapi32.lib"],
         "//conditions:default": ["-lpthread"],
     }),
     visibility = ["//visibility:public"],