x86/speculation: Add cpu_show_gds() prototype

The newly added function has two definitions but no prototypes:

drivers/base/cpu.c:605:16: error: no previous prototype for 'cpu_show_gds' [-Werror=missing-prototypes]

Add a declaration next to the other ones for this file to avoid the
warning.

Fixes: 8974eb5 ("x86/speculation: Add Gather Data Sampling mitigation")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Tested-by: Daniel Sneddon <daniel.sneddon@linux.intel.com>
Cc: stable@kernel.org
Link: https://lore.kernel.org/all/20230809130530.1913368-1-arnd%40kernel.org

Author: arndb

include/linux/cpu.h

@@ -72,6 +72,8 @@ extern ssize_t cpu_show_retbleed(struct device *dev,
 				 struct device_attribute *attr, char *buf);
 extern ssize_t cpu_show_spec_rstack_overflow(struct device *dev,
 					     struct device_attribute *attr, char *buf);
+extern ssize_t cpu_show_gds(struct device *dev,
+			    struct device_attribute *attr, char *buf);
 
 extern __printf(4, 5)
 struct device *cpu_device_create(struct device *parent, void *drvdata,