UBUNTU: [Packaging] fix invocation of header postinst hooks

BugLink: http://bugs.launchpad.net/bugs/1764794
OldBugLink: http://bugs.launchpad.net/bugs/1766391

There's a typo in the headers postinst which prevents triggering
of dkms builds on installation. Change this to use the correct
path, /etc/kernel/header_postinst.d.
Signed-off-by: Seth Forshee <seth.forshee@canonical.com>
Signed-off-by: Andy Whitcroft <apw@canonical.com>
Acked-by: Stefan Bader <stefan.bader@canonical.com>
Acked-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>
Signed-off-by: Stefan Bader <stefan.bader@canonical.com>

debian/templates/headers.postinst.in

@@ -7,9 +7,9 @@ if [ "$1" != configure ]; then
     exit 0
 fi
 
-if [ -d /etc/kernel/headers_postinst.d ]; then
+if [ -d /etc/kernel/header_postinst.d ]; then
     DEB_MAINT_PARAMS="$*" run-parts --report --exit-on-error --arg=$version \
-		/etc/kernel/headers_postinst.d
+		/etc/kernel/header_postinst.d
 fi
 
 exit 0