Fix compilation with windows.h from latest w32api

misc.h has complex logic (checking MAXSHORT is undefined etc.)
controlling if it includes assert.h or not.

Including windows.h from w32api 9.0.0 now trips over that, causing
assert.h to not be included, causing various errors, e.g.

In file included from ../include/cursor.h:53,
                 from ../include/dix.h:54,
                 from ../os/osdep.h:139,
                 from ../hw/xwin/winauth.c:40:
../include/privates.h: In function ‘dixGetPrivateAddr’:
../include/privates.h:121:5: error: implicit declaration of function ‘assert’ [-Werror=implicit-function-declaration]

Fix this by IWYU in privates.h
5 jobs for include-assert-h in 4 minutes and 45 seconds (queued for 6 seconds)