win32: Avoid compiler warning (WIN32_LEAN_AND_MEAN redefined)

configure adds the macro WIN32_LEAN_AND_MEAN to
QEMU_CFLAGS, and SDL_syswm.h defines it, too.

This results in a compiler warning (redefinition of
WIN32_LEAN_AND_MEAN in SDL_syswm.h. That warning prevents
compilations for win32 with warning = error).

Fix this by removing the definition of WIN32_LEAN_AND_MEAN
before including SDL_syswm.h.

[blauwirbel@gmail.com: rebased]
Signed-off-by: Stefan Weil <weil@mail.berlios.de>
Signed-off-by: Blue Swirl <blauwirbel@gmail.com>
