input: key_matrix: fix header inclusion

On Microblaze with device tree support enabled we run into
the error below.

I'm not sure, but I think that all source code should include
at least the common.h and just this fix the problem on
Microblaz architecture.

The error is:

In file included from key_matrix.c:29:
include/malloc.h:364: error: conflicting types for 'memset'
include/linux/string.h:71: error: previous declaration of 'memset' was here
include/malloc.h:365: error: conflicting types for 'memcpy'
include/linux/string.h:74: error: previous declaration of 'memcpy' was here

Signed-off-by: Stephan Linz <linz@li-pro.net>
CC: Bernie Thompson <bhthompson@chromium.org>
CC: Simon Glass <sjg@chromium.org>
CC: Tom Warren <twarren@nvidia.com>
CC: Michal Simek <monstr@monstr.eu>
Acked-by: Simon Glass <sjg@chromium.org>
Acked-by: Michal Simek <monstr@monstr.eu>