fix: Add git2/sys/config.h to systest headers, use extern "C" fn

Author: tecc

libgit2-sys/lib.rs

@@ -125,9 +125,9 @@ pub enum git_reference {}
 #[repr(C)]
 pub struct git_reference_iterator {
     pub db: *mut git_refdb,
-    pub next: Option<fn(*mut *mut git_reference, *mut git_reference_iterator) -> c_int>,
-    pub next_name: Option<fn(*mut *const c_char, *mut git_reference_iterator) -> c_int>,
-    pub free: Option<fn(*mut git_reference_iterator)>,
+    pub next: Option<extern "C" fn(*mut *mut git_reference, *mut git_reference_iterator) -> c_int>,
+    pub next_name: Option<extern "C" fn(*mut *const c_char, *mut git_reference_iterator) -> c_int>,
+    pub free: Option<extern "C" fn(*mut git_reference_iterator)>,
 }
 
 pub enum git_annotated_commit {}

systest/build.rs

@@ -16,6 +16,7 @@ fn main() {
         .header("git2/sys/repository.h")
         .header("git2/sys/cred.h")
         .header("git2/sys/email.h")
+        .header("git2/sys/config.h")
         .header("git2/cred_helpers.h")
         .type_name(|s, _, _| s.to_string());
     cfg.field_name(|_, f| match f {