build: Fix make distclean/distcheck

build: Fix make distclean/distcheck

In commit 0b5141d8f94 the sample-plugins got partially migrated to
automake.  But since it was not fully integrated within the full
standard build, the sample/sample-plugins/Makefile was not removed
by 'make distclean', which annoys 'make distcheck'.

The simplest way is just to explicitly enlist this Makefile in the list
of files 'make distclean' should remove.

Signed-off-by: David Sommerseth <davids@openvpn.net>
Acked-by: Gert Doering <gert@greenie.muc.de>
Message-Id: <20200916195616.30633-1-davids@openvpn.net>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg21026.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>
(cherry picked from commit 6345cea86bde14b22a8024cad5026a7d302db25c)