The problem arises during the build process when compiling `spice-core.c`. The compiler reports an implicit declaration of `qemu_close_to_socket`, indicating that this function is not declared before it's used. 

Looking into the source code, in other parts like `ui/qemu-cursor.c` and `ui/spice-display.c`, functions like `qemu_display_cursor_set` are used without prior declaration because they're declared as `extern` in `qemu-ui.h`. However, `qemu_close_to_socket` isn't declared anywhere, which leads to the error.

The fix is straightforward: declaring `qemu_close_to_socket` as an external function in the appropriate header file ensures it's visible to all parts of the code that use it. This declaration should be added to `ui/qemu-ui.h` to resolve the implicit declaration issue.
</think>

To fix this bug, we need to ensure that the function `qemu_close_to_socket` is properly declared before it is used in `spice-core.c`. The solution involves adding an external declaration of `qemu_close_to_socket` in the appropriate header file.

**Step-by-step explanation:**

1. **Identify the Missing Declaration:**  
   The error indicates that `qemu_close_to_socket` is not declared before it's used in `spice-core.c`.

2. **Locate the Function Definition:**  
   Search through the codebase to find where `qemu_close_to_socket` is defined.

3. **Add an External Declaration:**  
   Add `extern void qemu_close_to_socket(int fd);` in a header file that's included by `spice-core.c`.

**Implementation:**

Add the following line to `ui/qemu-ui.h`:

```c
extern void qemu_close_to_socket(int fd);
```

This declaration makes `qemu_close_to_socket` visible to other source files, resolving the implicit declaration error.

**Result:**  
After this change, rebuild the project. The compiler will no longer report the implicit function declaration error since the function is now properly declared before use.

```bash
# After making changes, rebuild
./configure --enable-sdl --enable-gtk --target-list=arm-softmmu,aarch64-softmmu
cd build && make
```

The build should proceed without errors related to `qemu_close_to_socket`.