Makefile: Another fix for make distcheck when using scan-build

It seems to run two configures with different parameters, which results in
some of the files not being cleaned up. So this hopefully fixes:

ERROR: files left in build directory after distclean:
./src/util/tcpwrap

Makefile.am

@@ -105,4 +105,5 @@ distcheck-hook:
 	  if ! rmdir scan-reports 2>/dev/null; then \
 	    exit 1; \
 	  fi; \
+	  make distclean; \
 	fi