r2d: fix build on mingw

Comment near strncpy explains kernel_cmdline does
not need to be 0-terminated.

Accordingly mark it as QEMU_NONSTRING.

Without this, gcc warns:
    'strncpy' specified bound 256 equals destination size

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

hw/sh4/r2d.c

@@ -220,7 +220,7 @@ static struct QEMU_PACKED
 
     char pad[232];
 
-    char kernel_cmdline[256];
+    char kernel_cmdline[256] QEMU_NONSTRING;
 } boot_params;
 
 static void r2d_init(MachineState *machine)