extcon: Remove unneeded extern keyword from extcon-provider.h

The commit tb7365587 ("extcon: Remove unneeded extern keyword
from extcon.h") removes the unneeded extern keyword from extcon header
file. But, The commit tb7365587

 has missed that deletes 'extern'
keyword from extcon-provider.h. So that it deletes extern keyword
from extcon-provider.h.

Signed-off-by: Chanwoo Choi <cw00.choi@samsung.com>
Link: https://lore.kernel.org/r/20200217104728.29330-1-cw00.choi@samsung.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>