HID: avoid sparse warning in HID_COMPAT_LOAD_DRIVER

Impact: include a prototype for the exported function in the macro

Fix about 20 of this warnings:

  drivers/hid/hid-a4tech.c:162:1: warning: symbol 'hid_compat_a4tech' was not declared. Should it be static?
Signed-off-by: NHannes Eder <hannes@hanneseder.net>
Signed-off-by: NJiri Kosina <jkosina@suse.cz>

include/linux/hid.h

@@ -793,6 +793,8 @@ dbg_hid(const char *fmt, ...)
 
 #ifdef CONFIG_HID_COMPAT
 #define HID_COMPAT_LOAD_DRIVER(name)	\
+/* prototype to avoid sparse warning */	\
+extern void hid_compat_##name(void);	\
 void hid_compat_##name(void) { }	\
 EXPORT_SYMBOL(hid_compat_##name)
 #else