kirkwood: set testing kernel version to 5.15

This patch configures kernel testing version for kirkwood target.

Compile tested: all
Run tested: Endian 4i

Signed-off-by: Pawel Dembicki <paweldembicki@gmail.com>

target/linux/kirkwood/Makefile

@@ -11,6 +11,7 @@ FEATURES:=usb nand squashfs ramdisk
 CPU_TYPE:=xscale
 
 KERNEL_PATCHVER:=5.10
+KERNEL_TESTING_PATCHVER:=5.15
 
 include $(INCLUDE_DIR)/target.mk