setjmp.h: add struct tag for sigjmp_buf (GCC C++ compatibility)

setjmp.h: add struct tag for sigjmp_buf (GCC C++ compatibility)

the anonymous struct typedef with array notation breaks with
GCC in C++ mode:

error: non-local function 'static<anonymous struct>
(& boost::signal_handler::jump_buffer())[1]' uses anonymous type

this is a known GCC issue, as search results for that error msg
suggest.

since this is hard to work around in the calling C++ code, a
fix in musl is preferable.