headers: Hide the contents of struct _iobuf in UCRT mode

This matches the public UCRT headers - the struct only contains one
single placeholder member.

Signed-off-by: Martin Storsjö <martin@martin.st>

mingw-w64-headers/crt/stdio.h

diff --git a/mingw-w64-headers/crt/stdio.h b/mingw-w64-headers/crt/stdio.h
index 89bfa14..c761779 100644
--- a/mingw-w64-headers/crt/stdio.h
+++ b/mingw-w64-headers/crt/stdio.h
@@ -22,6 +22,9 @@
 
 #ifndef _FILE_DEFINED
   struct _iobuf {
+#ifdef _UCRT
+    void *_Placeholder;
+#else
     char *_ptr;
     int _cnt;
     char *_base;
@@ -30,6 +33,7 @@
     int _charbuf;
     int _bufsiz;
     char *_tmpfname;
+#endif
   };
   typedef struct _iobuf FILE;
 #define _FILE_DEFINED