Fixed broken build of mksnapshot on Windows as there was warnings against...

src/mksnapshot.cc

Index: src/mksnapshot.cc
===================================================================
--- src/mksnapshot.cc	(revision 300)
+++ src/mksnapshot.cc	(working copy)
@@ -116,7 +116,7 @@
 static int WriteInternalSnapshotToFile(const char* filename,
                                        const char* str,
                                        int size) {
-  FILE* f = fopen(filename, "wb");
+  FILE* f = i::OS::FOpen(filename, "wb");
   if (f == NULL) {
     i::OS::PrintError("Cannot open file %s for reading.\n", filename);
     return 0;