Moved a bunch of definitions from gdi.h into a new gdi_private.h to
make sure we don't use them outside of gdi32.

diff --git a/objects/palette.c b/objects/palette.c
index 02966cf..c21e912 100644
--- a/objects/palette.c
+++ b/objects/palette.c
@@ -33,6 +33,7 @@
 #include "wownt32.h"
 #include "wine/winuser16.h"
 #include "gdi.h"
+#include "gdi_private.h"
 #include "palette.h"
 #include "wine/debug.h"
 #include "winerror.h"