pylibfdt: Don't emit warnings from swig generate C code

The swig generated code has a heap of warnings with the flags we usually
use.  These aren't helpful, since there's nothing we can do about them from
the dtc side.  So, just disable the warnings flags when compiling the
Python module.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
diff --git a/pylibfdt/Makefile.pylibfdt b/pylibfdt/Makefile.pylibfdt
index d20659c..647203f 100644
--- a/pylibfdt/Makefile.pylibfdt
+++ b/pylibfdt/Makefile.pylibfdt
@@ -15,6 +15,7 @@
 SETUPFLAGS += --quiet
 endif
 
+$(PYMODULE): WARNINGS =  # suppress warnings from generated code
 $(PYMODULE): $(PYLIBFDT_srcs) $(LIBFDT_archive) $(SETUP)
 	@$(VECHO) PYMOD $@
 	CFLAGS="$(CFLAGS) -Wno-error" $(PYTHON) $(SETUP) $(SETUPFLAGS) build_ext