[win32] Fix usage of GetFullPathName()

Diego Santa Cruz pointed out that we are using that API wrongly.
The forth argument is a pointer to a pointer.  Turns out we don't
need that arugment and it accepts NULL, so just pass that.

src/fcstr.c

@@ -968,10 +968,8 @@ FcStrCanonFilename (const FcChar8 *s)
 {
 #ifdef _WIN32
     FcChar8 full[FC_MAX_FILE_LEN + 2];
-    FcChar8 basename[FC_MAX_FILE_LEN + 2];
     int size = GetFullPathName (s, sizeof (full) -1,
-				full,
-				basename);
+				full, NULL);
 
     if (size == 0)
 	perror ("GetFullPathName");