Make it look for its plugin in its own installation directory.

Without this vmpk fails to start with "Unable to initialize all MIDI drivers".

--- a/library/rt/backendmanager.cpp
+++ b/library/rt/backendmanager.cpp
@@ -159,6 +159,7 @@ namespace rt {
         foreach(const QString& path, QCoreApplication::libraryPaths()) {
             d->appendDir( path + QDir::separator() + QSTR_DRUMSTICK, result );
         }
+        d->appendDir( "@out@/lib/" + QSTR_DRUMSTICK, result );
         return result;
     }