extcon: Mark extcon_get_edev_name() function as exported symbol

extcon_get_edev_name() function provides client driver to request
extcon dev's name. If extcon driver and client driver are compiled
as loadable modules, extcon_get_edev_name() function symbol is not
visible to client driver. Hence mark extcon_find_edev_name() function
as exported symbol.

Signed-off-by: Mayank Rana <mrana@codeaurora.org>
Signed-off-by: Chanwoo Choi <cw00.choi@samsung.com>