package/linux-headers: default to 5.15 series for _HEADERS_AS_KERNEL / KERNEL_LATEST_VERSION

Fixes #14611

BR2_KERNEL_HEADERS_LATEST_VERSION selects a 5.15.x series kernel (as that is
current LTS), but support for 5.16.x kernel headers has been added, so by
default the "custom kernel headers series" option ends up with 5.16.x (the
first option), leading to confusing errors:

Incorrect selection of kernel headers: expected 5.16.x, got 5.15.x

As a help, default to the correct 5.15.x series when this combination is
used.

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

package/linux-headers/Config.in.host

@@ -127,6 +127,8 @@ endif
 
 choice
 	bool "Custom kernel headers series"
+	default BR2_PACKAGE_HOST_LINUX_HEADERS_CUSTOM_5_15 if \
+		(BR2_KERNEL_HEADERS_AS_KERNEL && BR2_LINUX_KERNEL_LATEST_VERSION)
 	depends on BR2_KERNEL_HEADERS_VERSION || BR2_KERNEL_HEADERS_AS_KERNEL || \
 		   BR2_KERNEL_HEADERS_CUSTOM_TARBALL || BR2_KERNEL_HEADERS_CUSTOM_GIT
 	help