time: Define dummy functions for the generic sched clock

When we try to compile a clocksource driver with the COMPILE_TEST option,
we can't select the GENERIC_SCHED_CLOCK because the sched_clock() symbol
will be duplicated with the one defined for the x86.

In order to fix that, we don't select the GENERIC_SCHED_CLOCK in the
driver Kconfig's file but we define some empty functions for the different
symbols in order to prevent the unresolved ones.

This patch fixes the COMPILE_TEST option for the compile test coverage for
the clocksource drivers. Without this patch, we can't add the COMPILE_TEST
option for the clocksource drivers using the GENERIC_SCHED_CLOCK.
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>

include/linux/sched_clock.h

@@ -10,11 +10,17 @@
 
 #ifdef CONFIG_GENERIC_SCHED_CLOCK
 extern void sched_clock_postinit(void);
-#else
-static inline void sched_clock_postinit(void) { }
-#endif
 
 extern void sched_clock_register(u64 (*read)(void), int bits,
 				 unsigned long rate);
+#else
+static inline void sched_clock_postinit(void) { }
+
+static inline void sched_clock_register(u64 (*read)(void), int bits,
+					unsigned long rate)
+{
+	;
+}
+#endif
 
 #endif