staging: r8723au: remove externs and include needed header file

Remove extern declaration and add include of header file to get same.

Addresses checkpatch.pl:
WARNING: externs should be avoided in .c files

Signed-off-by: Alison Schofield <amsfield22@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>