MAINTAINERS: add missing extcon directory

Add the missing extcon directory to maintain them.  When using
get_maintainer.pl, the result should include the correct maintainer
information.
Signed-off-by: Chanwoo Choi <cw00.choi@samsung.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

MAINTAINERS

@@ -4207,7 +4207,10 @@ L:	linux-kernel@vger.kernel.org
 T:	git git://git.kernel.org/pub/scm/linux/kernel/git/chanwoo/extcon.git
 S:	Maintained
 F:	drivers/extcon/
+F:	include/linux/extcon/
+F:	include/linux/extcon.h
 F:	Documentation/extcon/
+F:	Documentation/devicetree/bindings/extcon/
 
 EXYNOS DP DRIVER
 M:	Jingoo Han <jingoohan1@gmail.com>