ftrace: Fix build warning

If CONFIG_SYSCTL and CONFIG_DYNAMIC_FTRACE is n, build warns:

kernel/trace/ftrace.c:7912:13: error: ‘is_permanent_ops_registered’ defined but not used [-Werror=unused-function]
 static bool is_permanent_ops_registered(void)
             ^~~~~~~~~~~~~~~~~~~~~~~~~~~
kernel/trace/ftrace.c:89:12: error: ‘last_ftrace_enabled’ defined but not used [-Werror=unused-variable]
 static int last_ftrace_enabled;
            ^~~~~~~~~~~~~~~~~~~

Move is_permanent_ops_registered() to ifdef block and mark last_ftrace_enabled as
__maybe_unused to fix this.

Fixes: 7cde53da38a3 ("ftrace: move sysctl_ftrace_enabled to ftrace.c")
Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Acked-by: Steven Rostedt (Google) <rostedt@goodmis.org>
Signed-off-by: Luis Chamberlain <mcgrof@kernel.org>