tracepoint: Use __used attribute definitions from compiler_attributes.h

tracepoint: Use __used attribute definitions from compiler_attributes.h

Just a small cleanup while I was touching this header.
compiler_attributes.h does feature detection of these __attributes__(())
and provides more concise ways to invoke them.

Link: https://lkml.kernel.org/r/20200730224555.2142154-3-ndesaulniers@google.com
Acked-by: Miguel Ojeda <miguel.ojeda.sandonis@gmail.com>
Signed-off-by: Nick Desaulniers <ndesaulniers@google.com>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>