headers: Remove RtlUnwind from winternl.h

Function RtlUnwind is already declared in the winnt.h header file.
And winternl.h includes windef.h which includes winnt.h.

Moreover declaration of RtlUnwind in winternl.h is incompatible with
the declaration in winnt.h and causes compile warning:

/usr/local/include/winternl.h:1130:14: warning: ‘RtlUnwind’ redeclared without dllimport attribute: previous dllimport ignored [-Wattributes]
   VOID NTAPI RtlUnwind (PVOID TargetFrame,PVOID TargetIp,PEXCEPTION_RECORD ExceptionRecord,PVOID ReturnValue);
              ^~~~~~~~~

Signed-off-by: LIU Hao <lh_mouse@126.com>