mender_x86_64_efi_defconfig: specify 5.18 kernel headers

The defconfig uses a 5.18.4 kernel, so specify 5.18 kernel headers now that
is available.

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

configs/mender_x86_64_efi_defconfig

@@ -2,7 +2,7 @@
 BR2_x86_64=y
 
 # Toolchain
-BR2_PACKAGE_HOST_LINUX_HEADERS_CUSTOM_5_17=y
+BR2_PACKAGE_HOST_LINUX_HEADERS_CUSTOM_5_18=y
 
 # System
 BR2_TARGET_GENERIC_GETTY_PORT="ttyS0"