sh: syscall: Add missing forward declaration for sys_cacheflush()

arch/sh/kernel/sys_sh.c:58:16: warning: no previous prototype for ‘sys_cacheflush’ [-Wmissing-prototypes]
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: John Paul Adrian Glaubitz <glaubitz@physik.fu-berlin.de>
Link: https://lore.kernel.org/r/b1c218986fb77d78303626b43c97925809045018.1709326528.git.geert+renesas@glider.beSigned-off-by: John Paul Adrian Glaubitz <glaubitz@physik.fu-berlin.de>

arch/sh/include/asm/syscalls.h

@@ -8,6 +8,7 @@ asmlinkage int old_mmap(unsigned long addr, unsigned long len,
 asmlinkage long sys_mmap2(unsigned long addr, unsigned long len,
 			  unsigned long prot, unsigned long flags,
 			  unsigned long fd, unsigned long pgoff);
+asmlinkage int sys_cacheflush(unsigned long addr, unsigned long len, int op);
 
 #include <asm/syscalls_32.h>