util: Suppress build assertions when building with sparse.

util: Suppress build assertions when building with sparse.

sparse simply doesn't like our build assertions on packed structures.
It seems that its ideas about struct packing are different from GCC's:

../lib/cfm.h:50:1: error: invalid bitfield width, -1.
../lib/packets.h:206:1: error: invalid bitfield width, -1.
../lib/packets.h:213:1: error: invalid bitfield width, -1.
../lib/packets.h:367:1: error: invalid bitfield width, -1.

sparse isn't generating code so we don't really care how it lays out
structures.  We might as well just skip the assertions, as done here.