debian/collectd-core.overrides: update build-depends-on-obsolete-package usage

Boy, this is annoying!

From: Michael Prokop <mprokop@sipwise.com>

debian/collectd-core.overrides

@@ -13,5 +13,5 @@ collectd-core: capitalization-error-in-description python Python
 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 # dpatch is obsolete but we can't deprecated it yet to avoid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
+# a big diff to Debian's "upstream" package
+collectd source: build-depends-on-obsolete-package dpatch