ftrace/fgraph: fix increased missing-prototypes warnings

After commit e999995c ("ftrace: cleanup ftrace_graph_caller enable
and disable") merged into the linux-next tree, the kernel test robot
(lkp@intel.com) has send out report that there are increased missing-prototypes
warnings caused by that commit.

COMPILER_INSTALL_PATH=$HOME/0day COMPILER=gcc-11.3.0 make.cross W=1 \
	O=build_dir ARCH=sh SHELL=/bin/bash kernel/trace/
warning: no previous prototype for 'ftrace_enable_ftrace_graph_caller' [-Wmissing-prototypes]
warning: no previous prototype for 'ftrace_disable_ftrace_graph_caller' [-Wmissing-prototypes]
warning: no previous prototype for 'ftrace_return_to_handler' [-Wmissing-prototypes]
warning: no previous prototype for 'ftrace_graph_sleep_time_control' [-Wmissing-prototypes]

BTW there are so many missing-prototypes warnings if build kernel with "W=1".

The increased warnings for 'ftrace_[enable,disable]_ftrace_graph_caller'
is caused by CONFIG_FUNCTION_GRAPH_TRACER && !CONFIG_DYNAMIC_FTRACE,
so the declarations in <linux/ftrace.h> can't be seen in fgraph.c.

And this warning can't reproduce on x86_64 since x86_64 select
HAVE_FUNCTION_GRAPH_TRACER only when DYNAMIC_FTRACE, so fgraph.c will
always see the declarations in <linux/ftrace.h>.

This patch fix the increased warnings by put the definitions in
CONFIG_DYNAMIC_FTRACE although there are no real problems exist.
Signed-off-by: Chengming Zhou <zhouchengming@bytedance.com>
Acked-by: Steven Rostedt (Google) <rostedt@goodmis.org>
Link: https://lore.kernel.org/r/20220506032737.23375-1-zhouchengming@bytedance.comSigned-off-by: Catalin Marinas <catalin.marinas@arm.com>

kernel/trace/fgraph.c

@@ -30,6 +30,7 @@ int ftrace_graph_active;
 /* Both enabled by default (can be cleared by function_graph tracer flags */
 static bool fgraph_sleep_time = true;
 
+#ifdef CONFIG_DYNAMIC_FTRACE
 /*
  * archs can override this function if they must do something
  * to enable hook for graph tracer.
@@ -47,6 +48,7 @@ int __weak ftrace_disable_ftrace_graph_caller(void)
 {
 	return 0;
 }
+#endif
 
 /**
  * ftrace_graph_stop - set to permanently disable function graph tracing