extcon: add EXPORT_SYMBOL_GPL for exported functions

Added EXPORT_SYMBOL_GPL() for extcon_register_interest and
extcon_register_notifier in order to avoid undefined reference
error when building the consumer modules of extcon as _modules_.

Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>
Signed-off-by: Lokesh Vutla <lokeshvutla@ti.com>
Signed-off-by: Chanwoo Choi <cw00.choi@samsung.com>
Signed-off-by: Myungjoo Ham <myungjoo.ham@samsung.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>