ssh-agent: require ppx_cstruct >= 5.2.0
ssh-agent: require ppx_cstruct >= 5.2.0

ppx_cstruct prior to 5.2.0 used OCaml AST 4.04 (or earlier), and we need
at least 4.08.

nberth/opam-repository,emillon/opam-repository,pirbo/opam-repository,kkirstein/opam-repository,pqwy/opam-repository,kkirstein/opam-repository,astrada/opam-repository,emillon/opam-repository,hannesm/opam-repository,pqwy/opam-repository,Leonidas-from-XIV/opam-repository,andersfugmann/opam-repository,ocaml/opam-repository,yallop/opam-repository,hannesm/opam-repository,ocaml/opam-repository,Leonidas-from-XIV/opam-repository,codinuum/opam-repository,jhwoodyatt/opam-repository,pirbo/opam-repository,yallop/opam-repository,codinuum/opam-repository,nberth/opam-repository,astrada/opam-repository,fpottier/opam-repository,djs55/opam-repository,fpottier/opam-repository,andersfugmann/opam-repository,djs55/opam-repository,jhwoodyatt/opam-repository