MAINTAINERS: Add include directory to MFD file patterns

Ensure that get_maintainer.pl does the right thing for header only
changes.

Signed-off-by: Mark Brown <[e-mail]>
Signed-off-by: Samuel Ortiz <[e-mail]>

MAINTAINERS

···
 T:	git git://git.kernel.org/pub/scm/linux/kernel/git/sameo/mfd-fixes.git
 S:	Supported
 F:	drivers/mfd/
+F:	include/linux/mfd/
 
 MULTIMEDIA CARD (MMC), SECURE DIGITAL (SD) AND SDIO SUBSYSTEM
 M:	Chris Ball <[e-mail]>