Fix a stringop-truncation GCC warning

```
minetest/src/filesys.cpp:312:10: warning: ‘char* strncpy(char*, const char*, size_t)’ specified bound 10000 equals destination size [-Wstringop-truncation]
   strncpy(argv_data[2], path.c_str(), 10000);
```

src/filesys.cpp

@@ -309,7 +309,7 @@ bool RecursiveDelete(const std::string &path)
 		strcpy(argv_data[0], "/bin/rm");
 #endif
 		strcpy(argv_data[1], "-rf");
-		strncpy(argv_data[2], path.c_str(), 10000);
+		strncpy(argv_data[2], path.c_str(), sizeof(argv_data[2]) - 1);
 		char *argv[4];
 		argv[0] = argv_data[0];
 		argv[1] = argv_data[1];