ubootRock3C: enable strictDeps

Added for fixing failing CI: nixpkgs-vet (NPV-164)

buildUBoot does not enable strictDeps by default and the
existing ubootRock* packages predate the rule, so the new ubootRock3C
attribute trips the lint.

Set strictDeps = true on the ubootRock3C derivation only to keep blast
radius minimal.

pkgs/misc/uboot/default.nix

@@ -794,6 +794,7 @@ in
   ubootRock3C = buildUBoot {
     defconfig = "rock-3c-rk3566_defconfig";
     extraMeta.platforms = [ "aarch64-linux" ];
+    strictDeps = true;
     env = {
       BL31 = "${armTrustedFirmwareRK3568}/bl31.elf";
       ROCKCHIP_TPL = rkbin.TPL_RK3566;