[SDK] Introduce winbase_undoc.h for undocumented exported kernel32 definitions (#8019)

This header is based on the following files from the official
Windows 10.0.10240.0 PSDK, a copy of which can be found at:
- https://github.com/tpn/winsdk-10/blob/master/Include/10.0.10240.0/um/minwin/winbasep.h
- https://github.com/tpn/winsdk-10/blob/master/Include/10.0.10240.0/um/minwin/wbasek.h

win32ss/pch.h

@@ -35,9 +35,10 @@
 typedef struct _SECURITY_ATTRIBUTES SECURITY_ATTRIBUTES, *LPSECURITY_ATTRIBUTES;
 #define MAKEINTATOM(i) (LPWSTR)((ULONG_PTR)((WORD)(i)))
 #define WINBASEAPI
-#define STARTF_USESHOWWINDOW 1
-#define STARTF_USESIZE 2
-#define STARTF_USEPOSITION 4
+#define STARTF_USESHOWWINDOW    0x00000001
+#define STARTF_USESIZE          0x00000002
+#define STARTF_USEPOSITION      0x00000004
+#define STARTF_USEHOTKEY        0x00000200
 #include <windef.h>
 
 // Needed because windef.h messes up CDECL for whatever

win32ss/user/ntuser/main.c

@@ -8,18 +8,12 @@
 
 #include <win32k.h>
 #include <napi.h>
+#include <winbase_undoc.h>
 
 #define NDEBUG
 #include <debug.h>
 #include <kdros.h>
 
-#ifndef STARTF_USEHOTKEY
-#define STARTF_USEHOTKEY    0x0200
-#endif
-#ifndef STARTF_SHELLPRIVATE
-#define STARTF_SHELLPRIVATE 0x0400
-#endif
-
 HANDLE hModuleWin;
 
 NTSTATUS ExitProcessCallback(PEPROCESS Process);