git-shell build error

Hello,

If NO_SETENV is defined, git-shell cannot be built.
Simply adding the 'compat/setenv.o' to the make rule fixes the problem.

Regards,

[master] ~/srcs/git[30]$ LANG= make
     LINK git-shell
Undefined                       first referenced
  symbol                             in file
gitsetenv                           exec_cmd.o
ld: fatal: Symbol referencing errors. No output written to git-shell
collect2: ld returned 1 exit status
make: *** [git-shell] Error 1