bpftool: Define _GNU_SOURCE only once

_GNU_SOURCE is defined in several source files for bpftool, but only one
of them takes the precaution of checking whether the value is already
defined. Add #ifndef for other occurrences too.

This is in preparation for the support of disassembling JIT-ed programs
with LLVM, with $(llvm-config --cflags) passing -D_GNU_SOURCE as a
compilation argument.

Signed-off-by: Quentin Monnet <quentin@isovalent.com>
Tested-by: Niklas Söderlund <niklas.soderlund@corigine.com>
Acked-by: Song Liu <song@kernel.org>
Link: https://lore.kernel.org/r/20221025150329.97371-2-quentin@isovalent.com


Signed-off-by: Alexei Starovoitov <ast@kernel.org>