Fix __sprintfkf prototype in libgcc.

Fix __sprintfkf prototype in libgcc.

The prototype of __sprintfkf in _sprintfkf.h did not match the function in
_sprintfkf.c.  This patch fixes the prototype.  I also included the
_sprintfkf.h file in _sprintfkf.c to make sure the prototype is correct and to
eliminate a warning about declaring the function without a previous
declaration.

libgcc/
2021-03-01  Michael Meissner  <meissner@linux.ibm.com>

	* config/rs6000/_sprintfkf.h (__sprintfkf): Fix prototype to match
	the function.
	* config/rs6000/_sprintfkf.c: Include _sprintfkf.h.