selftests/bpf: Add RESOLVE_BTFIDS dependency to bpf_testmod.ko

bpf_testmod.ko sometimes fails to build from a clean checkout:

    BTF [M] linux/tools/testing/selftests/bpf/bpf_testmod/bpf_testmod.ko
    /bin/sh: 1: linux-build//tools/build/resolve_btfids/resolve_btfids: not found

The reason is that RESOLVE_BTFIDS may not yet be built. Fix by adding a
dependency.

Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Acked-by: Jiri Olsa <jolsa@kernel.org>
Link: https://lore.kernel.org/bpf/20230403172935.1553022-1-iii@linux.ibm.com