mb/system76/lemp9: Use absolute path for _GPE

_GPE cannot be anywhere but at the root of the ACPI namespace.

This change ensures that is always the case.

Tested on lemp9, GPE still in correct location.

Signed-off-by: Jeremy Soller <jeremy@system76.com>
Change-Id: Ib31683b06e61da4b1859cd939c36879cebf4c03c
Reviewed-on: https://review.coreboot.org/c/coreboot/+/43630
Reviewed-by:  Felix Singer <felixsinger@posteo.net>
Reviewed-by: Michael Niewöhner
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

src/mainboard/system76/lemp9/acpi/mainboard.asl

@@ -9,6 +9,6 @@ Scope (\_SB) {
 	#include "sleep.asl"
 }
 
-Scope (_GPE) {
+Scope (\_GPE) {
 	#include "gpe.asl"
 }