curses: Fix compiler warnings (Mingw-w64 redefinition of macro KEY_EVENT)

curses: Fix compiler warnings (Mingw-w64 redefinition of macro KEY_EVENT)

For builds with Mingw-w64 as it is included in Cygwin, there are two
header files which define KEY_EVENT with different values.

This results in lots of compiler warnings like this one:

  CC      vl.o
In file included from /qemu/include/ui/console.h:340:0,
                 from /qemu/vl.c:76:
/usr/i686-w64-mingw32/sys-root/mingw/include/curses.h:1522:0: warning: "KEY_EVENT" redefined
 #define KEY_EVENT 0633  /* We were interrupted by an event */

In file included from /usr/share/mingw-w64/include/windows.h:74:0,
                 from /usr/share/mingw-w64/include/winsock2.h:23,
                 from /qemu/include/sysemu/os-win32.h:29,
                 from /qemu/include/qemu/osdep.h:100,
                 from /qemu/vl.c:24:
/usr/share/mingw-w64/include/wincon.h:101:0: note: this is the location of the previous definition
 #define KEY_EVENT 0x1

QEMU only uses the KEY_EVENT macro from wincon.h.
Therefore we can undefine the macro coming from curses.h.

The explicit include statement for curses.h in ui/curses.c is not needed
and was removed.

Those two modifications fix the redefinition warnings.

Signed-off-by: Stefan Weil <sw@weilnetz.de>