Define attach_type_name in common.c instead of main.h so it is only
defined once. This leads to a slight decrease in the binary size of
bpftool.

Before:

   text	   data	    bss	    dec	    hex	filename
 399024	  11168	1573160	1983352	 1e4378	bpftool

After:

   text	   data	    bss	    dec	    hex	filename
 398256	  10880	1573160	1982296	 1e3f58	bpftool
Signed-off-by: Tobias Klauser <tklauser@distanz.ch>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Reviewed-by: Quentin Monnet <quentin@isovalent.com>
Link: https://lore.kernel.org/bpf/20200624143154.13145-1-tklauser@distanz.ch