.gitlab-ci.yml: regenerate for new defconfig

Commit a8fac3fcfc2c71 introduced qemu_ppc_mac99_defconfig but forgot to
add it to .gitlab-ci.yml.

Do that now.

Fixes: https://gitlab.com/buildroot.org/buildroot/-/jobs/263733672

Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>

.gitlab-ci.yml

@@ -268,6 +268,7 @@ qemu_ppc64_e5500_defconfig: { extends: .defconfig }
 qemu_ppc64_pseries_defconfig: { extends: .defconfig }
 qemu_ppc64le_pseries_defconfig: { extends: .defconfig }
 qemu_ppc_g3beige_defconfig: { extends: .defconfig }
+qemu_ppc_mac99_defconfig: { extends: .defconfig }
 qemu_ppc_mpc8544ds_defconfig: { extends: .defconfig }
 qemu_ppc_virtex_ml507_defconfig: { extends: .defconfig }
 qemu_riscv32_virt_defconfig: { extends: .defconfig }