Add a libipchecksum that contains the C IP checksum code.

This is useful for test code that wants to statically link the IP
checksum code. This is not currently possible because libnetutils
is vendor_available and making it available for static linking is
discouraged.

Test: m libnetutils libipchecksum
Change-Id: Ic7f1864902581b8a5e3626860565b78c168bdb7f