spawn:posix: Rename the field to ENVP to avoid a build issue.

* src/spawn-posix.c (struct gpgrt_spawn_actions): Rename the field to
ENVP.
(my_exec, _gpgrt_spawn_actions_set_environ): Follow the change.

• Signed-off-by: Sachin T <sachin.t@ibm.com>
• Co-authored-by: NIIBE Yutaka <gniibe@fsij.org>