libsshpp: include required <string> header for std::string
libsshpp: include required <string> header for std::string

Signed-off-by: Petar Koretic <0bdc63247af490de2e3f3da15a8939ec28fc7d99@sartura.hr>
Reviewed-by: Andreas Schneider <5be00ddc76278cf6077f5047ca3384a88460c671@cryptomilk.org>

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