init/calibrate.c: provide proper prototype

Sparse issues a warning:

    CHECK   init/calibrate.c
  init/calibrate.c:271:28: warning: symbol 'calibration_delay_done' was not declared. Should it be static?

The actual issue is that it's a __weak symbol that archs can override
(in fact, ARM does so), but no prototype is provided.  Let's provide one
to prevent surprises.

Link: http://lkml.kernel.org/r/18827.1548750938@turing-police.cc.vt.edu
Signed-off-by: Valdis Kletnieks <valdis.kletnieks@vt.edu>
Reviewed-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

include/linux/delay.h

@@ -55,6 +55,7 @@ static inline void ndelay(unsigned long x)
 
 extern unsigned long lpj_fine;
 void calibrate_delay(void);
+void __attribute__((weak)) calibration_delay_done(void);
 void msleep(unsigned int msecs);
 unsigned long msleep_interruptible(unsigned int msecs);
 void usleep_range(unsigned long min, unsigned long max);