package/wireguard-linux-compat: add missing comment about kernel dependency

Add a comment when kernel is not enabled (missing since the addition of
the package in commit de591c5c3a93a40d049a32bd79c30cc66daf5fb2)

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>

package/wireguard-linux-compat/Config.in

@@ -21,6 +21,9 @@ config BR2_PACKAGE_WIREGUARD_LINUX_COMPAT
 
 	  https://www.wireguard.com
 
+comment "wireguard-linux-compat needs a Linux kernel to be built"
+	depends on !BR2_LINUX_KERNEL
+
 comment "wireguard-linux-compat needs a toolchain w/ headers >= 3.10"
 	depends on BR2_LINUX_KERNEL
 	depends on !BR2_TOOLCHAIN_HEADERS_AT_LEAST_3_10