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 <festevam@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

configs/imx6slevk_defconfig

@@ -12,6 +12,7 @@ BR2_TARGET_UBOOT=y
 BR2_TARGET_UBOOT_BOARDNAME="mx6slevk"
 BR2_TARGET_UBOOT_CUSTOM_VERSION=y
 BR2_TARGET_UBOOT_CUSTOM_VERSION_VALUE="2021.10"
+BR2_TARGET_UBOOT_NEEDS_OPENSSL=y
 BR2_TARGET_UBOOT_FORMAT_IMX=y
 BR2_LINUX_KERNEL=y
 BR2_LINUX_KERNEL_CUSTOM_VERSION=y