bpfilter: fix user mode helper cross compilation

Use $(OBJDUMP) instead of literal 'objdump' to avoid
using host toolchain when cross compiling.

Fixes: 421780fd4983 ("bpfilter: fix build error")
Signed-off-by: Matteo Croce <mcroce@redhat.com>
Reported-by: Stefano Brivio <sbrivio@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/bpfilter/Makefile

@@ -22,7 +22,7 @@ endif
 quiet_cmd_copy_umh = GEN $@
       cmd_copy_umh = echo ':' > $(obj)/.bpfilter_umh.o.cmd; \
       $(OBJCOPY) -I binary \
-          `LC_ALL=C objdump -f net/bpfilter/bpfilter_umh \
+          `LC_ALL=C $(OBJDUMP) -f net/bpfilter/bpfilter_umh \
           |awk -F' |,' '/file format/{print "-O",$$NF} \
           /^architecture:/{print "-B",$$2}'` \
       --rename-section .data=.init.rodata $< $@