x86/error_inject: Make just_return_func() globally visible

With link time optimizations enabled, I get a link failure:

  ./ccLbOEHX.ltrans19.ltrans.o: In function `override_function_with_return':
  <artificial>:(.text+0x7f3): undefined reference to `just_return_func'

Marking the symbol .globl makes it work as expected.
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Masami Hiramatsu <mhiramat@kernel.org>
Acked-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Alexei Starovoitov <ast@kernel.org>
Cc: Josef Bacik <jbacik@fb.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Nicolas Pitre <nico@linaro.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Fixes: 540adea3 ("error-injection: Separate error-injection from kprobe")
Link: http://lkml.kernel.org/r/20180202145634.200291-3-arnd@arndb.deSigned-off-by: Ingo Molnar <mingo@kernel.org>

arch/x86/lib/error-inject.c

@@ -7,6 +7,7 @@ asmlinkage void just_return_func(void);
 
 asm(
 	".type just_return_func, @function\n"
+	".globl just_return_func\n"
 	"just_return_func:\n"
 	"	ret\n"
 	".size just_return_func, .-just_return_func\n"