Makefile: Add target for generating tags

Running "ctags -R" takes a long time because of git submodules:

  [penberg@localhost osv]$ time ctags -R

  real	0m35.076s
  user	0m18.770s
  sys	0m3.929s

Add a "make tags" target that's much faster to run:

  [penberg@localhost osv]$ time make tags
  find . -name "*.cc" -o -name "*.hh" -o -name "*.h" -o -name "*.c" | ctags -L -

  real	0m0.980s
  user	0m1.033s
  sys	0m0.158s

Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>