drivers: base: Add prototype declaration to the header file

Add prototype declaration of function memory_block_size_bytes() to
the header file include/linux/memory.h.

This eliminates the following warning in memory.c:
drivers/base/memory.c:87:1: warning: no previous prototype for ‘memory_block_size_bytes’ [-Wmissing-prototypes]
Signed-off-by: Rashika Kheria <rashika.kheria@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

include/linux/memory.h

@@ -35,6 +35,7 @@ struct memory_block {
 };
 
 int arch_get_memory_phys_device(unsigned long start_pfn);
+unsigned long __weak memory_block_size_bytes(void);
 
 /* These states are exposed to userspace as text strings in sysfs */
 #define	MEM_ONLINE		(1<<0) /* exposed to userspace */