MIPS: Fix vmlinuz build when ZBOOT is selected

vmlinuz is not built by default for platforms using
COMPRESSION_FNAME (e.g. Malta) due to an erroneous
check on ZBOOT

Signed-off-by: Daniel Sabogal <dsabogalcc@gmail.com>
Reviewed-by: James Hogan <jhogan@kernel.org>
Cc: Ralf Baechle <ralf@linux-mips.org>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/18466/


Signed-off-by: James Hogan <jhogan@kernel.org>