bpf: fix warning from basename()

bpf: fix warning from basename()

The function basename() expects a mutable character string,
which now causes a warning:

bpf_legacy.c: In function ‘bpf_load_common’:
bpf_legacy.c:975:38: warning: passing argument 1 of ‘__xpg_basename’ discards ‘const’ qualifier from pointer target type [-Wdiscarded-qualifiers]
  975 |                          basename(cfg->object), cfg->mode == EBPF_PINNED ?
      |                                   ~~~^~~~~~~~
In file included from bpf_legacy.c:21:
/usr/include/libgen.h:34:36: note: expected ‘char *’ but argument is of type ‘const char *’
   34 | extern char *__xpg_basename (char *__path) __THROW;

Fixes: f20ff2f19552 ("bpf: keep parsed program mode in struct bpf_cfg_in")
Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>