Define prog_type_name in prog.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
 401032	  11936	1573160	1986128	 1e4e50	bpftool

After:

   text	   data	    bss	    dec	    hex	filename
 399024	  11168	1573160	1983352	 1e4378	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/20200624143124.12914-1-tklauser@distanz.ch