function_graph: Make fgraph_do_direct static key static

function_graph: Make fgraph_do_direct static key static

The static branch key "fgraph_do_direct" was not declared static but is
only used in one file. Change it to a static variable.

Link: https://lore.kernel.org/linux-trace-kernel/20240605202708.936515302@goodmis.org
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Fixes: cc60ee813b503 ("function_graph: Use static_call and branch to optimize entry function")
Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202406051711.dS1sQZ9n-lkp@intel.com/
Acked-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>