libbpf: make sure bpf headers are c++ include-able

Wrap headers in extern "C", to turn off C++ mangling.
This simplifies including libbpf in c++ and linking against it.

v2 changes:
* do the same for btf.h

v3 changes:
* test_libbpf.cpp to test for possible future c++ breakages

Signed-off-by: Stanislav Fomichev <sdf@google.com>
Acked-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>

tools/lib/bpf/bpf.h

@@ -27,6 +27,10 @@
 #include <stdbool.h>
 #include <stddef.h>
 
+#ifdef __cplusplus
+extern "C" {
+#endif
+
 #ifndef LIBBPF_API
 #define LIBBPF_API __attribute__((visibility("default")))
 #endif
@@ -132,4 +136,9 @@ LIBBPF_API int bpf_load_btf(void *btf, __u32 btf_size, char *log_buf,
 LIBBPF_API int bpf_task_fd_query(int pid, int fd, __u32 flags, char *buf,
 				 __u32 *buf_len, __u32 *prog_id, __u32 *fd_type,
 				 __u64 *probe_offset, __u64 *probe_addr);
+
+#ifdef __cplusplus
+} /* extern "C" */
+#endif
+
 #endif /* __LIBBPF_BPF_H */