Handle __builtin_ms_va_list.

Handle __builtin_ms_va_list.

For Win64 compiles Wine does
 #ifndef __ms_va_list
 # if defined(__x86_64__) && defined (__GNUC__)
 #  define __ms_va_list __builtin_ms_va_list
 #  define __ms_va_start(list,arg) __builtin_ms_va_start(list,arg)
 #  define __ms_va_end(list) __builtin_ms_va_end(list)
 # else

Wouldn't be as bad if sparse cannot handle those but it trips over
WINBASEAPI DWORD WINAPI FormatMessageA(DWORD,LPCVOID,DWORD,DWORD,LPSTR,DWORD,__ms_va_list*);
WINBASEAPI DWORD WINAPI FormatMessageW(DWORD,LPCVOID,DWORD,DWORD,LPWSTR,DWORD,__ms_va_list*);
producing this errors for basically every file:
wine/include/winbase.h:1546:96: error: Expected ) in function declarator
wine/include/winbase.h:1546:96: error: got *
wine/include/winbase.h:1547:97: error: Expected ) in function declarator
wine/include/winbase.h:1547:97: error: got *

Signed-off-by: Michael Stefaniuc <mstefaniuc@gmail.com>
Signed-off-by: Christopher Li <sparse@chrisli.org>