[gdb] Fix compilation error in event-top.c

[gdb] Fix compilation error in event-top.c

       CXX    event-top.o
     In file included from d:\usr\include\winsock2.h:69,
		      from ./../gdbsupport/gdb_select.h:30,
		      from event-top.c:43:
     event-top.c: In function 'fd_set* fd_copy(fd_set*, const fd_set*, int)':
     event-top.c:1279:22: error: invalid conversion from 'const fd_set*' to 'fd_set*' [-fpermissive]
      1279 |     if (FD_ISSET (i, src))
	   |                      ^
	   |                      |
	   |                      const fd_set*
     d:\usr\include\winsock.h:164:50: note:   initializing argument 2 of 'int __FD_ISSET(SOCKET, fd_set*)'
       164 | __CRT_ALIAS int __FD_ISSET( SOCKET __fd, fd_set *__set )
	   |                                          ~~~~~~~~^~~~~

  Use an explicit type cast to prevent this.

      * gdb/event-top.c (fd_copy): Type-cast in call to FD_ISSET.