tools/resolve_btfids: Do not print any commands when building silently

When building with 'make -s', there is some output from resolve_btfids:

$ make -sj"$(nproc)" oldconfig prepare
  MKDIR     .../tools/bpf/resolve_btfids/libbpf/
  MKDIR     .../tools/bpf/resolve_btfids//libsubcmd
  LINK     resolve_btfids

Silent mode means that no information should be emitted about what is
currently being done. Use the $(silent) variable from Makefile.include
to avoid defining the msg macro so that there is no information printed.

Fixes: fbbb68de ("bpf: Add resolve_btfids tool to resolve BTF IDs in ELF object")
Signed-off-by: Nathan Chancellor <nathan@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20220201212503.731732-1-nathan@kernel.org