.gitignore: add build dirs and shim_cert.h
.gitignore: add build dirs and shim_cert.h

This adds stuff that only ever gets made as an artifact of building
(though build*/ generally doesn't, as of this commit.)

Signed-off-by: Peter Jones <2e0a021e603479ff81838c00c3b770daca4e0214@redhat.com>

rhinstaller/shim,rhinstaller/shim,rhinstaller/shim