Fix 'make depend' for out of the source tree build.

diff --git a/configure.in b/configure.in
index b896b921..fca9d5c 100644
--- a/configure.in
+++ b/configure.in
@@ -1201,6 +1201,7 @@
 dlls/dinput/keyboard \
 dlls/dinput/mouse \
 dlls/kernel/messages \
+dlls/user/dde \
 dlls/user/resources \
 dlls/wineps/data \
 "