Provide a conversion function from utf8 to utf16.

Add missing #include <linux/types.h> in include/charset.h.
Remove superfluous #include <common.h> in lib/charset.c.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Signed-off-by: Alexander Graf <agraf@suse.de>

We'll eventually want these in a few places in efi_loader, and also
vsprintf.

Signed-off-by: Rob Clark <robdclark@gmail.com>