selftests/bpf: Filter out _GNU_SOURCE when compiling test_cpp

Jakub reports build failures when merging linux/master with net tree:

CXX      test_cpp
In file included from <built-in>:454:
<command line>:2:9: error: '_GNU_SOURCE' macro redefined [-Werror,-Wmacro-redefined]
    2 | #define _GNU_SOURCE
      |         ^
<built-in>:445:9: note: previous definition is here
  445 | #define _GNU_SOURCE 1

The culprit is commit cc937dad ("selftests: centralize -D_GNU_SOURCE= to
CFLAGS in lib.mk") which unconditionally added -D_GNU_SOUCE to CLFAGS.
Apparently clang++ also unconditionally adds it for the C++ targets [0]
which causes a conflict. Add small change in the selftests makefile
to filter it out for test_cpp.

Not sure which tree it should go via, targeting bpf for now, but net
might be better?

0: https://stackoverflow.com/questions/11670581/why-is-gnu-source-defined-by-default-and-how-to-turn-it-off



Signed-off-by: Stanislav Fomichev <sdf@fomichev.me>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Acked-by: Jiri Olsa <jolsa@kernel.org>
Link: https://lore.kernel.org/bpf/20240725214029.1760809-1-sdf@fomichev.me