configs/imx6slevk: Select BR2_TARGET_UBOOT_NEEDS_OPENSSL

configs/imx6slevk: Select BR2_TARGET_UBOOT_NEEDS_OPENSSL

Select BR2_TARGET_UBOOT_NEEDS_OPENSSL to fix the following build
error:

include/image.h:1178:12: fatal error: openssl/evp.h: No such file or directory
 1178 | #  include <openssl/evp.h>
      |            ^~~~~~~~~~~~~~~

After the update of u-boot in commit 9a04bbec1cd59b (configs/imx6slevk: bump
U-Boot).

Fixes:
- https://gitlab.com/buildroot.org/buildroot/-/jobs/1946650628
Signed-off-by: Fabio Estevam <[email protected]>
Signed-off-by: Peter Korsgaard <[email protected]>