staging: rtl8723bs: Add missing include <linux/of.h> to fix compile error

As reported by Stephen Rothwell when merging staging-next,
drivers/staging/rtl8723bs/core/rtw_ieee80211.c does not compile due
to of_get_property not being declared.

Explicitly include <linux/of.h> to fix this compile error.

Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>