acpi_sigcmp defined in the header uses memcmp, but no included headers
pull in memcmp, which can lead to compile errors in new code, depending
on include order. Fix this.

Reported-by: @healytpk, https://github.com/saschahauer/barebox/issues/7
Signed-off-by: Ahmad Fatoum <ahmad@a3f.at>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>