MIPS: Fix build error for uncompressed non-plain vmlinux kernels

Seen on rm200_defconfig for example:

  CC      arch/mips/boot/compressed/decompress.o
/home/ralf/src/linux/upstream-linus/arch/mips/boot/compressed/decompress.c: In function ‘decompress_kernel’:
/home/ralf/src/linux/upstream-linus/arch/mips/boot/compressed/decompress.c:116: error: implicit declaration of function ‘decompress’
make[3]: *** [arch/mips/boot/compressed/decompress.o] Error 1
make[2]: *** [vmlinuz.ecoff] Error 2
make[1]: *** [sub-make] Error 2
make: *** [all] Error 2

Signed-off-by: Ralf Baechle <ralf@linux-mips.org>