dh: Use $(filter) rather than $(findstring)

dh: Use $(filter) rather than $(findstring)

$(findstring) can match partial strings and so is unreliable when a
package builds several binary packages and one package contains the
name of another package within its name.  In these cases,
$(findstring) can return a partial match which leads to problems
(performing unwanted actions which could lead to build failure, for
example).

$(filter) matches the entire string in the wordlist, so is a
reliable replacement for $(findstring).

Signed-off-by: Roger Leigh <rleigh@debian.org>