dtc: Update the build rule for pylibfdt

Some versions of make complain about using a grouped target without a
recipe:

.../pylibfdt/Makefile:36: *** grouped targets must provide a recipe.  Stop.

Fix this by adding a dummy recipe.
Signed-off-by: NSimon Glass <sjg@chromium.org>

scripts/dtc/pylibfdt/Makefile

@@ -34,6 +34,7 @@ rebuild: $(src)/setup.py $(PYLIBFDT_srcs)
 	fi
 
 $(obj)/_libfdt.so $(obj)/libfdt.py &: rebuild
+	@:
 
 always += _libfdt.so libfdt.py