config/odroidc2: force host kernel version to 5.7

as the current kernel version used for this board is 5.7,
make sure that the host kernel version is the same to prevent
build failure due to version miss match.

Fixes:
https://gitlab.com/buildroot.org/buildroot/-/jobs/779210973

Signed-off-by: Dagg Stompler <daggs@gmx.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>

configs/odroidc2_defconfig

@@ -1,6 +1,9 @@
 # Architecture
 BR2_aarch64=y
 
+# Linux headers same as kernel, a 5.7 series
+BR2_PACKAGE_HOST_LINUX_HEADERS_CUSTOM_5_7=y
+
 # System
 BR2_TARGET_GENERIC_HOSTNAME="odroidc2"
 BR2_TARGET_GENERIC_GETTY_PORT="ttyAML0"