Add attribute warn_unused_result to compiler.h and dt_expand_node

This would have thrown a compiler warning for cf8def9 rather than
failing at runtime.

Signed-off-by: Stewart Smith <stewart@linux.vnet.ibm.com>
Liberally sprinkle attribute((warn_unused_result)) around the place

None of these actually produce any warnings. My next patch will.

Signed-off-by: Stewart Smith <stewart@linux.vnet.ibm.com>