cpu/speculation: Add prototype for cpu_show_srbds()

0-day is not happy that there is no prototype for cpu_show_srbds():

  drivers/base/cpu.c:565:16: error: no previous prototype for 'cpu_show_srbds'

Fixes: 7e5b3c26 ("x86/speculation: Add Special Register Buffer Data Sampling (SRBDS) mitigation")
Reported-by: Nkernel test robot <lkp@intel.com>
Signed-off-by: NGuenter Roeck <linux@roeck-us.net>
Signed-off-by: NBorislav Petkov <bp@suse.de>
Link: https://lkml.kernel.org/r/20200617141410.93338-1-linux@roeck-us.net

include/linux/cpu.h

@@ -64,6 +64,7 @@ extern ssize_t cpu_show_tsx_async_abort(struct device *dev,
 					char *buf);
 extern ssize_t cpu_show_itlb_multihit(struct device *dev,
 				      struct device_attribute *attr, char *buf);
+extern ssize_t cpu_show_srbds(struct device *dev, struct device_attribute *attr, char *buf);
 
 extern __printf(4, 5)
 struct device *cpu_device_create(struct device *parent, void *drvdata,