include/wdm.h: Fix -Wsign-compare warning in RtlExtendedMagicDivide()

x86_64-w64-mingw32-gcc throws following -Wsign-compare warning when
compiling 64-bit NT kernel drivers which include ddk/ntddk.h file:

  ddk/wdm.h: In function ‘RtlExtendedMagicDivide’:
  ddk/wdm.h:9531:32: warning: operand of ?: changes signedness from ‘long long int’ to ‘ULONG64’ {aka ‘long long unsigned int’} due to unsignedness of other operand [-Wsign-compare]
     ret.QuadPart = Pos ? ret64 : -(LONG64)ret64;
                                  ^~~~~~~~~~~~~~
Member ret.QuadPart is of type LONGLONG, so explicitly cast both parts of
ret64 ternary operator to LONG64.

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