xtensa: Fix broken allmodconfig build

xtansa allmodbuild fails with:

arch/xtensa/kernel/xtensa_ksyms.c:129:1: error: '_mcount' undeclared here (not in a function)
make[2]: *** [arch/xtensa/kernel/xtensa_ksyms.o] Error 1
make[1]: *** [arch/xtensa/kernel] Error 2

The breakage is due to commit 478ba61a

 (xtensa: add static function tracer
support) which exports _mcount without declaring it.

Cc: Max Filippov <jcmvbkbc@gmail.com>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Chris Zankel <chris@zankel.net>