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: NGuenter Roeck <linux@roeck-us.net>
Signed-off-by: NChris Zankel <chris@zankel.net>

arch/xtensa/kernel/xtensa_ksyms.c

@@ -25,6 +25,7 @@
 #include <asm/io.h>
 #include <asm/page.h>
 #include <asm/pgalloc.h>
+#include <asm/ftrace.h>
 #ifdef CONFIG_BLK_DEV_FD
 #include <asm/floppy.h>
 #endif