selftests/bpf: Fix C++ compile error from missing _Bool type

While building, bpftool makes a skeleton from test_core_extern.c, which
itself includes <stdbool.h> and uses the 'bool' type. However, the skeleton
test_core_extern.skel.h generated *does not* include <stdbool.h> or use the
'bool' type, instead using the C-only '_Bool' type. Compiling test_cpp.cpp
with g++ 12.3 for mips64el/musl-libc then fails with error:

  In file included from test_cpp.cpp:9:
  test_core_extern.skel.h:45:17: error: '_Bool' does not name a type
     45 |                 _Bool CONFIG_BOOL;
        |                 ^~~~~

This was likely missed previously because glibc uses a GNU extension for
<stdbool.h> with C++ (#define _Bool bool), not supported by musl libc.

Normally, a C fragment would include <stdbool.h> and use the 'bool' type,
and thus cleanly work after import by C++. The ideal fix would be for
'bpftool gen skeleton' to output the correct type/include supporting C++,
but in the meantime add a conditional define as above.

Fixes: 7c8dce4 ("bpftool: Make skeleton C code compilable with C++ compiler")
Signed-off-by: Tony Ambardar <tony.ambardar@gmail.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/6fc1dd28b8bda49e51e4f610bdc9d22f4455632d.1722244708.git.tony.ambardar@gmail.com

tools/testing/selftests/bpf/test_cpp.cpp

@@ -6,6 +6,10 @@
 #include <bpf/libbpf.h>
 #include <bpf/bpf.h>
 #include <bpf/btf.h>
+
+#ifndef _Bool
+#define _Bool bool
+#endif
 #include "test_core_extern.skel.h"
 #include "struct_ops_module.skel.h"