MIPS: ftrace: Add missing CONFIG_DYNAMIC_FTRACE

arch_ftrace_update_code and ftrace_modify_all_code are only
available if CONFIG_DYNAMIC_FTRACE is selected.

Fixes the following build problem on MIPS randconfig:

arch/mips/kernel/ftrace.c: In function 'arch_ftrace_update_code':
arch/mips/kernel/ftrace.c:31:2: error: implicit declaration of function
'ftrace_modify_all_code' [-Werror=implicit-function-declaration]
Signed-off-by: Markos Chandras <markos.chandras@imgtec.com>
Acked-by: Steven J. Hill <Steven.Hill@imgtec.com>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/5435/Signed-off-by: Ralf Baechle <ralf@linux-mips.org>

arch/mips/kernel/ftrace.c

@@ -25,12 +25,16 @@
 #define MCOUNT_OFFSET_INSNS 4
 #endif
 
+#ifdef CONFIG_DYNAMIC_FTRACE
+
 /* Arch override because MIPS doesn't need to run this from stop_machine() */
 void arch_ftrace_update_code(int command)
 {
 	ftrace_modify_all_code(command);
 }
 
+#endif
+
 /*
  * Check if the address is in kernel space
  *