Fix Windows build.

Only two days in, and I'm already really regretting putting this code
somewhere that builds for Mac OS and Windows...

Change-Id: I576ee4a9e647e10dc2d47c7e1e38aedee2bf404c
diff --git a/libutils/file.cpp b/libutils/file.cpp
index bfd1f55..5b1ce88 100644
--- a/libutils/file.cpp
+++ b/libutils/file.cpp
@@ -70,6 +70,7 @@
   return false;
 }
 
+#if !defined(_WIN32)
 bool android::WriteStringToFile(const std::string& content, const std::string& path,
                                 mode_t mode, uid_t owner, gid_t group) {
   int fd = TEMP_FAILURE_RETRY(open(path.c_str(),
@@ -85,6 +86,7 @@
   }
   return CleanUpAfterFailedWrite(path);
 }
+#endif
 
 bool android::WriteStringToFile(const std::string& content, const std::string& path) {
   int fd = TEMP_FAILURE_RETRY(open(path.c_str(),