Declare 'ioarg' (from ser-tcp.c:try_connect) as 'u_long' when on Windows (and unbreak...

Declare 'ioarg' (from ser-tcp.c:try_connect) as 'u_long' when on Windows (and unbreak build on mingw32)

When building GDB on mingw32, it fails with:

  ../../binutils-gdb/gdb/ser-tcp.c: In function 'int try_connect(const addrinfo*, unsigned int*)':
  ../../binutils-gdb/gdb/ser-tcp.c:176:25: error: invalid conversion from 'int*' to 'u_long* {aka long unsigned int*}' [-fpermissive]
     ioctl (sock, FIONBIO, &ioarg);
			   ^~~~~~
  In file included from ../../binutils-gdb/gdb/serial.h:23:0,
		   from ../../binutils-gdb/gdb/ser-tcp.c:21:
  /usr/x86_64-w64-mingw32/sys-root/mingw/include/winsock2.h:977:34: note:   initializing argument 3 of 'int ioctlsocket(SOCKET, long int, u_long*)'
     WINSOCK_API_LINKAGE int WSAAPI ioctlsocket(SOCKET s,__LONG32 cmd,u_long *argp);
				    ^~~~~~~~~~~
  make[2]: *** [Makefile:1610: ser-tcp.o] Error 1

The problem happens because the IPv6
commit (c7ab0aef11d91b637bf091aa9176b8dc4aadee46) wrongly removed the
code responsible for declaring 'ioarg' with a different type if
building for Windows.  This patch restores that.

gdb/ChangeLog:
2018-07-12  Sergio Durigan Junior  <sergiodj@redhat.com>

	* ser-tcp.c (try_connect): Declare 'ioarg' as 'u_long' if building
	on Windows.