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 <broonie@linaro.org>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>

MAINTAINERS

@@ -5437,6 +5437,7 @@ T:	git git://git.kernel.org/pub/scm/linux/kernel/git/sameo/mfd-next.git
 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 <cjb@laptop.org>