tools: bpftool: fix potential format truncation

GCC 7 complains:

xlated_dumper.c: In function ‘print_call’:
xlated_dumper.c:179:10: warning: ‘%s’ directive output may be truncated writing up to 255 bytes into a region of size between 249 and 253 [-Wformat-truncation=]
     "%+d#%s", insn->off, sym->name);

Add a bit more space to the buffer so it can handle the entire
string and integer without truncation.

Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>