Fix Texas Instruments compiler warning

Texas Instruments compiler produces this warning for unused template
member functions:

	"pugixml.cpp", line 253: warning #179-D: function
	"pugi::impl::<unnamed>::auto_deleter<T>::release [with
	T=pugi::impl::<unnamed>::xml_stream_chunk<char>]" was declared but
	never referenced

As far as I can tell, this is a compiler issue - these functions should
not be instantiated in the first place; while it's possible to rework
the code to work around this, the changes would be fragile. It seems
best to just disable this warning - we've seen something similar on SNC
(which appears to use the same frontend!..).

Fixes #182.