[niepce] Fix warning in ffi

[Hubert Figuière <hub src gnome org>]

commit 9c10235442cf900692d94f9b6d7f2ea2e9fa8468
Author: Hubert Figuière <hub figuiere net>
Date:   Fri Jul 2 22:30:13 2021 -0400

    Fix warning in ffi

 crates/npc-fwk/src/utils/files.rs | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)
---
diff --git a/crates/npc-fwk/src/utils/files.rs b/crates/npc-fwk/src/utils/files.rs
index f385c42..3bb84d6 100644
--- a/crates/npc-fwk/src/utils/files.rs
+++ b/crates/npc-fwk/src/utils/files.rs
@@ -116,7 +116,7 @@ pub unsafe extern "C" fn fwk_file_list_delete(l: *mut FileList) {
 #[no_mangle]
 pub unsafe extern "C" fn fwk_file_list_get_files_from_directory(
     dir: *const c_char,
-    filter: Option<fn(*mut gio_sys::GFileInfo) -> bool>,
+    filter: Option<extern "C" fn(*mut gio_sys::GFileInfo) -> bool>,
 ) -> *mut FileList {
     let cstr = CStr::from_ptr(dir);
     match filter {