riscv: Fix build dependency for loader

The Makefile addition for the flat image loader missed an obj prefix.

For most parallel builds this worked out fine, but with -j1 the dependency
wasn't fulfilled and thus fails:

arch/riscv/boot/loader.S: Assembler messages:
arch/riscv/boot/loader.S:7: Error: file not found: arch/riscv/boot/Image

Fixes: 405fe7aa0dba ("riscv: provide a flat image loader")
Cc: Christoph Hellwig <[e-mail]>
Signed-off-by: Olof Johansson <[e-mail]>
Reviewed-by: Anup Patel <[e-mail]>
Signed-off-by: Paul Walmsley <[e-mail]>

arch/riscv/boot/Makefile

···
 $(obj)/Image.gz: $(obj)/Image FORCE
 	$(call if_changed,gzip)
 
-loader.o: $(src)/loader.S $(obj)/Image
+$(obj)/loader.o: $(src)/loader.S $(obj)/Image
 
 $(obj)/loader: $(obj)/loader.o $(obj)/Image $(obj)/loader.lds FORCE
 	$(Q)$(LD) -T $(obj)/loader.lds -o $@ $(obj)/loader.o