Add libelf-dev to headers template

Per stevenpusser, if this package is not already previously installed,
the headers won't install it for you and any sources depending on it
will fail to build.

Forum post: https://techpatterns.com/forums/about2629.html

linux-liquorix/debian/templates/control.headers.in

@@ -1,5 +1,5 @@
 Package: linux-headers-@abiname@@localversion@
-Depends: ${shlibs:Depends}, ${misc:Depends}
+Depends: ${shlibs:Depends}, ${misc:Depends}, libelf-dev
 Conflicts: r8168-dkms
 Description: Header files for Linux @abiname@@localversion@
  This package provides the architecture-specific kernel header files