os: include <assert.h> in ospoll.c

Fixes build failure in mingw-cross-build:

../os/ospoll.c: In function ‘ospoll_destroy’:
../os/ospoll.c:266:9: error: implicit declaration of function ‘assert’
 [-Werror=implicit-function-declaration]
  266 |         assert (ospoll->num == 0);
      |         ^~~~~~
../os/ospoll.c:59:1: note: ‘assert’ is defined in header ‘<assert.h>’;
 did you forget to ‘#include <assert.h>’?
   58 | #include "xserver_poll.h"
  +++ |+#include <assert.h>
   59 | #define POLL            1
../os/ospoll.c:266:9: warning: nested extern declaration of ‘assert’
 [-Wnested-externs]
  266 |         assert (ospoll->num == 0);
      |         ^~~~~~

Signed-off-by: Alan Coopersmith <alan.coopersmith@oracle.com>
Part-of: <https://gitlab.freedesktop.org/xorg/xserver/-/merge_requests/2141>

os/ospoll.c

@@ -24,6 +24,7 @@
 #include <dix-config.h>
 #endif
 
+#include <assert.h>
 #include <stdlib.h>
 #include <unistd.h>
 #include <X11/X.h>