Add V8_EXPORT_PRIVATE to v8::internal::GetRandomMmapAddr()

To unbreak the component build of unittests on Mac after #48124.

Bug: chromium:768094
Change-Id: I0e0f4ade0e19a71554f68e7050c525376f125ae5
Reviewed-on: https://chromium-review.googlesource.com/682094Reviewed-by: Bill Budge <bbudge@chromium.org>
Commit-Queue: Hans Wennborg <hans@chromium.org>
Cr-Commit-Position: refs/heads/master@{#48146}

src/allocation.h

@@ -160,7 +160,7 @@ bool AlignedAllocVirtualMemory(size_t size, size_t alignment, void* hint,
                                VirtualMemory* result);
 
 // Generate a random address to be used for hinting mmap().
-void* GetRandomMmapAddr();
+V8_EXPORT_PRIVATE void* GetRandomMmapAddr();
 
 }  // namespace internal
 }  // namespace v8