env: solve compilation error in SPL

Solve compilation issue when cli_simple.o is used in SPL
and CONFIG_SPL_ENV_SUPPORT is not defined.

env/built-in.o:(.data.env_htab+0xc): undefined reference to `env_flags_validate'
u-boot/scripts/Makefile.spl:384: recipe for target 'spl/u-boot-spl' failed
make[2]: *** [spl/u-boot-spl] Error 1
u-boot/Makefile:1649: recipe for target 'spl/u-boot-spl' failed
make[1]: *** [spl/u-boot-spl] Error 2

Signed-off-by: Patrick Delaunay <patrick.delaunay@st.com>