Fix obvious syntax error that somehow works fine on MSVC and GCC 3.x-win32.

src/google/protobuf/compiler/subprocess.h

@@ -76,7 +76,7 @@ class Subprocess {
 #ifdef _WIN32
   // Given an error code, returns a human-readable error message.  This is
   // defined here so that CommandLineInterface can share it.
-  static string Subprocess::Win32ErrorMessage(DWORD error_code);
+  static string Win32ErrorMessage(DWORD error_code);
 #endif
 
  private: