Export functions in io_win32.h in win DLL build

Previous code duplicates io_win32.cc in sources of all the libraries
that include io_win32.h header.

src/google/protobuf/stubs/io_win32.h

@@ -54,19 +54,20 @@ namespace protobuf {
 namespace internal {
 namespace win32 {
 
-FILE* fopen(const char* path, const char* mode);
-int access(const char* path, int mode);
-int chdir(const char* path);
-int close(int fd);
-int dup(int fd);
-int dup2(int fd1, int fd2);
-int mkdir(const char* path, int _mode);
-int open(const char* path, int flags, int mode = 0);
-int read(int fd, void* buffer, size_t size);
-int setmode(int fd, int mode);
-int stat(const char* path, struct _stat* buffer);
-int write(int fd, const void* buffer, size_t size);
-std::wstring testonly_path_to_winpath(const std::string& path);
+LIBPROTOBUF_EXPORT FILE* fopen(const char* path, const char* mode);
+LIBPROTOBUF_EXPORT int access(const char* path, int mode);
+LIBPROTOBUF_EXPORT int chdir(const char* path);
+LIBPROTOBUF_EXPORT int close(int fd);
+LIBPROTOBUF_EXPORT int dup(int fd);
+LIBPROTOBUF_EXPORT int dup2(int fd1, int fd2);
+LIBPROTOBUF_EXPORT int mkdir(const char* path, int _mode);
+LIBPROTOBUF_EXPORT int open(const char* path, int flags, int mode = 0);
+LIBPROTOBUF_EXPORT int read(int fd, void* buffer, size_t size);
+LIBPROTOBUF_EXPORT int setmode(int fd, int mode);
+LIBPROTOBUF_EXPORT int stat(const char* path, struct _stat* buffer);
+LIBPROTOBUF_EXPORT int write(int fd, const void* buffer, size_t size);
+LIBPROTOBUF_EXPORT std::wstring testonly_path_to_winpath(
+    const std::string& path);
 
 }  // namespace win32
 }  // namespace internal