built_in: Include stddef.h to avoid redefinition of offsetof() macro
The stddef.h header might define the offsetof() macro unconditionally,
leading to the macro being redefined if built_in.h is included _before_
stddef.h. This will lead sparse to complain as follows:
/usr/lib/gcc/x86_64-linux-gnu/4.8//include/stddef.h:413:9: warning: preprocessor token offsetof redefined
./built_in.h:151:10: this was the original definition
Fix this by explicitly including stddef.h in built_in.h
Signed-off-by: Tobias Klauser <tklauser@distanz.ch>