debian/collectd-core.overrides: add build-depends-on-obsolete-package dpatch

Supposed to fix lintian error:

| E: collectd source: build-depends-on-obsolete-package build-depends: dpatch

From: Michael Prokop <mprokop@sipwise.com>

debian/collectd-core.overrides

@@ -11,3 +11,7 @@ collectd-core: capitalization-error-in-description python Python
 # directory. According to the Java guys the path name and the ABI is stable
 # though ...
 collectd-core: binary-or-shlib-defines-rpath usr/lib/collectd/java.so /usr/lib/jvm/java-6-openjdk/jre/lib/*
+
+# dpatch is obsolete but we can't deprecated it yet to avoid
+# diff to upstream
+collectd: build-depends-on-obsolete-package build-depends: dpatch