built_in: Prevent __const_hton{l,s}/__const_ntoh{l,s} redefinition

Prefer the system header definitions. Found using smatch, which produced the
following output:

/usr/include/linux/byteorder/little_endian.h:14:9: warning: preprocessor token __constant_htonl redefined
built_in.h:234:11: this was the original definition
/usr/include/linux/byteorder/little_endian.h:15:9: warning: preprocessor token __constant_ntohl redefined
built_in.h:237:11: this was the original definition
/usr/include/linux/byteorder/little_endian.h:16:9: warning: preprocessor token __constant_htons redefined
built_in.h:240:11: this was the original definition
/usr/include/linux/byteorder/little_endian.h:17:9: warning: preprocessor token __constant_ntohs redefined
built_in.h:243:11: this was the original definition

Signed-off-by: Tobias Klauser <tklauser@distanz.ch>