update gitignore file
update gitignore file

The libssh library by default does not allow in-source build (with cmake
MacroEnsureOutOfSourceBuild macro). The INSTALL file (implicitly) suggests
creating a build directory. So lets add build to list of git ignore files to
avoid complaints from git.

Signed-off-by: Luka Perkov <fe6d8aafe911b69ec3eecb89e7941038c0d25a22@sartura.hr>
Reviewed-by: Andreas Schneider <5be00ddc76278cf6077f5047ca3384a88460c671@cryptomilk.org>

DouglasHeriot/libssh,wangshawn/libssh,kedazo/libssh,mwgoldsmith/ssh,taikoo/libssh,robxu9/libssh,DouglasHeriot/libssh,kedazo/libssh,DouglasHeriot/libssh,pouete/libssh,elastichosts/libssh,pouete/libssh,sebadoom/libssh,pouete/libssh,Distrotech/libssh,elastichosts/libssh,taikoo/libssh,robxu9/libssh,DouglasHeriot/libssh,elastichosts/libssh,sebadoom/libssh,pouete/libssh,taikoo/libssh,Distrotech/libssh,wangshawn/libssh,Distrotech/libssh,sebadoom/libssh,kedazo/libssh,nviennot/libssh,nviennot/libssh,taikoo/libssh,mwgoldsmith/libssh,wangshawn/libssh,robxu9/libssh,nviennot/libssh,robxu9/libssh,mwgoldsmith/ssh,mwgoldsmith/libssh,kedazo/libssh,Distrotech/libssh,mwgoldsmith/libssh,sebadoom/libssh,nviennot/libssh,mwgoldsmith/libssh,mwgoldsmith/ssh,mwgoldsmith/ssh,wangshawn/libssh