configs/orangepi_one_plus: fix kernel headers option

Contrary to what the comment in the defconfig says, the
orangepi_one_plus_defconfig was not using the "same as kernel" option
for kernel headers, but really selecting explicitly Linux 4.18
headers, independently from the kernel version.

Except that in the mean time, BR2_KERNEL_HEADERS_4_18 has been
removed, causing a build failure due to the legacy checking:

Makefile.legacy:9: *** "You have legacy configuration in your .config! Please check your configuration.".  Stop.

This commit fixes that by using the proper
BR2_PACKAGE_HOST_LINUX_HEADERS_CUSTOM_4_18 option.

Fixes:

  https://gitlab.com/buildroot.org/buildroot/-/jobs/158295166



Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>