chardev: move headers to include/chardev

So they are all in one place. The following patch will move serial &
parallel declarations to the respective headers.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>

chardev/char-null.c

@@ -22,7 +22,7 @@
  * THE SOFTWARE.
  */
 #include "qemu/osdep.h"
-#include "sysemu/char.h"
+#include "chardev/char.h"
 
 static void null_chr_open(Chardev *chr,
                           ChardevBackend *backend,