libntfs: Fix -Wsizeof-array-decay

Fix sizeof() parameter since sizeof(buffer - 1) returns size of pointer,
not size of array. Pointed out by clang.

Change-Id: Ib0be8a9de65c24a787e5817ea70a1cfc1c14b18f
Reviewed-on: https://review.haiku-os.org/743
Reviewed-by: Adrien Destugues <pulkomandy@pulkomandy.tk>

src/add-ons/kernel/file_systems/ntfs/libntfs/logging.c

@@ -491,7 +491,7 @@ int ntfs_log_handler_fprintf(const char *function, const char *file,
 #ifndef __HAIKU__
 	ret += vfprintf(stream, format, args);
 #else
-	ret += vsnprintf(buffer, sizeof(buffer - 1), format, args);
+	ret += vsnprintf(buffer, sizeof(buffer), format, args);
 	dprintf("%s", buffer);
 #endif