Clean doxygen_sqlite3.db

Generated by newer versions of Doxygen when built with sqlite3.
diff --git a/doc/Makefile.am b/doc/Makefile.am
index 296c4dd..31fddab 100644
--- a/doc/Makefile.am
+++ b/doc/Makefile.am
@@ -36,6 +36,7 @@
 	$(RM) -r latex
 	$(RM) -r man
 	$(RM) doxygen-build.stamp
+	$(RM) doxygen_sqlite3.db
 
 uninstall-local:
 	$(RM) -r $(DESTDIR)$(docdir)/html