setserial: don't build setserial.cat

Fixes http://autobuild.buildroot.net/results/c73/c736933d4110865008f2d1e8b531b6e392c68285/



Don't use 'make all', as that also builds setserial.cat which needs nroff on
the build machine (and isn't needed anyway as it doesn't get installed /
referenced from anywhere).

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>