Expose protected v8::Platform::SystemClockTimeMillis

This allows the embedder to use a shared library build
even if they use this method.

R=ulan@chromium.org

Change-Id: I613a6e5eb82b494128fb95dc89a0b73639ac5ca2
Reviewed-on: https://chromium-review.googlesource.com/c/1456042Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Yang Guo <yangguo@chromium.org>
Cr-Commit-Position: refs/heads/master@{#59455}

include/v8-platform.h

@@ -430,7 +430,7 @@ class Platform {
    * since epoch. Useful for implementing |CurrentClockTimeMillis| if
    * nothing special needed.
    */
-  static double SystemClockTimeMillis();
+  V8_EXPORT static double SystemClockTimeMillis();
 };
 
 }  // namespace v8