ARM: Fix RiscPC decompressor build errors

arch/arm/boot/compressed/decompress.o: In function `do_decompress':
decompress.c:(.text+0x26e8): undefined reference to `error'
decompress.c:(.text+0x2760): undefined reference to `error'
decompress.c:(.text+0x27d8): undefined reference to `error'
decompress.c:(.text+0x2824): undefined reference to `error'
decompress.c:(.text+0x28f0): undefined reference to `error'

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>