tools/env/fw_env.h: remove env.h

tools/env/fw_env.h: remove env.h

As reported by Nicolas Carrier on the Buildroot mailing list [1],
there is a new build issue while building a program which interacts with
the u-boot environment. This program uses the headers of the ubootenv
library provided by uboot-tools.

This is a recent change from uboot [2] adding "#include <env.h>" to
fw_env.h. Adding env.h require a board configuration to build since
it also include compiler.h (and others uboot internal includes).

env.h include seems not needed since env_set() is not used in fw_env tool.

Nicolas removed env.h from fw_env tool and fixed it's build issue.

This problem is present since uboot v2019.10.

[1] http://lists.busybox.net/pipermail/buildroot/2020-April/280307.html
[2] https://gitlab.denx.de/u-boot/u-boot/-/commit/9fb625ce05539fe6876a59ce1dcadb76b33c6f6e

Reported-by: Nicolas Carrier <nicolas.carrier@orolia.com>
Signed-off-by: Romain Naour <romain.naour@gmail.com>