dialog.c: give ctrl->fileselect.filter an opaque type.

The values of that field in a Control structure are already
platform-dependent: you're only supposed to set them in cross-platform
code by using #defined names that each platform will define
differently.

Now I need the _type_ as well as the values to be opaque, because I'm
about to make a change on Windows that turns it into a wide character
string instead of a char string.

windows/platform.h

@@ -286,6 +286,7 @@ void write_aclip(HWND hwnd, int clipboard, char *, int);
  * these strings are of exactly the type needed to go in
  * `lpstrFilter' in an OPENFILENAME structure.
  */
+typedef const char *FILESELECT_FILTER_TYPE;
 #define FILTER_KEY_FILES ("PuTTY Private Key Files (*.ppk)\0*.ppk\0" \
                               "All Files (*.*)\0*\0\0\0")
 #define FILTER_WAVE_FILES ("Wave Files (*.wav)\0*.WAV\0" \