Fixed for libgit2 1.8 (#920)

Fixed for libgit2:1.8.0-2

```
The error functions and enumeration values have been renamed for consistency. The giterr_ functions and values prefix have been renamed to be prefixed with git_error_; similarly, the GITERR_ constants have been renamed to be prefixed with GIT_ERROR_. The old enumerations and macros will be retained for backward compatibility for the foreseeable future.
```
https://github.com/libgit2/libgit2/blob/main/docs/changelog.md

src/Git2/Exception.cc

@@ -2,6 +2,7 @@
 
 #include "../Rustify.hpp"
 
+#include <git2/deprecated.h>
 #include <git2/errors.h>
 #include <git2/version.h>
 
@@ -25,7 +26,7 @@ Exception::Exception() {
   if (const git_error* error = git_error_last(); error != nullptr) {
     this->msg += error->message;
     this->cat = static_cast<git_error_t>(error->klass);
-    git_error_clear();
+    giterr_clear();
   }
 }