Fix v8 Windows GN build.

The paths for some of the Windows-specific files was incorrect.

R=jochen@chromium.org

Review URL: https://codereview.chromium.org/402693002

Patch from Brett Wilson <brettw@chromium.org>.

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@22455 ce2b1a6d-e550-0410-aec6-3dcde31c8c00

BUILD.gn

@@ -1059,9 +1059,9 @@ source_set("v8_libbase") {
     # TODO(jochen): Add support for cygwin.
     sources += [
       "src/base/platform/platform-win32.cc",
-      "src/base/platform/win32-headers.h",
-      "src/base/platform/win32-math.cc",
-      "src/base/platform/win32-math.h",
+      "src/base/win32-headers.h",
+      "src/base/win32-math.cc",
+      "src/base/win32-math.h",
     ]
 
     defines += [ "_CRT_RAND_S" ]  # for rand_s()