Fixed typo. File is called Doxydep and not DoxyDep.

[[Imported from SVN: r729]]

doc/doxygen/Makefile.am

@@ -98,7 +98,7 @@ FORCE:
 include $(top_srcdir)/am/webstuff
 
 clean-local:
-	if test -e DoxyDep; then rm -rf html html-dist; fi
+	if test -e Doxydep; then rm -rf html html-dist; fi
 	rm -rf latex *~
 
 CLEANFILES = $(WHAT) $(DOXYGENTAG) $(DOXYGENHEADER)