envtools: make sure version/timestamp header file are available

With commit 84d46e7e ("tools: env: allow to print U-Boot version")
the fw_env utilities need the version.h header file. Building only
the envtools in a pristine build directory will fail due to missing
header files.

Make sure the header files are a dependency of the envtools target.

Fixes: 84d46e7e

 ("tools: env: allow to print U-Boot version")
Signed-off-by: Stefan Agner <stefan.agner@toradex.com>
Tested-by: Peter Robinson <pbrobinson@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>