configs/mx6cubox: 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>
      |            ^~~~~~~~~~~~~~~

Fixes:
- https://gitlab.com/buildroot.org/buildroot/-/jobs/1915006257
Signed-off-by: Fabio Estevam <festevam@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>

configs/mx6cubox_defconfig

@@ -39,6 +39,7 @@ BR2_TARGET_UBOOT_NEEDS_PYLIBFDT=y
 BR2_TARGET_UBOOT_FORMAT_IMG=y
 BR2_TARGET_UBOOT_SPL=y
 BR2_TARGET_UBOOT_SPL_NAME="SPL"
+BR2_TARGET_UBOOT_NEEDS_OPENSSL=y
 
 # required tools to create the SD card image
 BR2_PACKAGE_HOST_DOSFSTOOLS=y