bpftool: Fix C++ additions to skeleton

Mark C++-specific T::open() and other methods as static inline to avoid
symbol redefinition when multiple files use the same skeleton header in
an application.

Fixes: bb8ffe61 ("bpftool: Add C++-specific open/load/etc skeleton wrappers")
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20220216233540.216642-1-andrii@kernel.org