ARM: 9291/1: decompressor: simplify the path to the top vmlinux

With commit 8debed3e ("kbuild: export top-level LDFLAGS_vmlinux
only to scripts/Makefile.vmlinux") applied, we no longer see the error
message while building the ARM zImage, but we do not have a good reason
to complicate the file path either.

'$(obj)/../../../../vmlinux' is canonicalized to 'vmlinux'.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>