[frontend] Fix nightly update checker (#3444)

Splitting

Signed-off-by: crueter <crueter@eden-emu.dev>
Reviewed-on: https://git.eden-emu.dev/eden-emu/eden/pulls/3444

src/frontend_common/update_checker.h

@@ -19,4 +19,5 @@ typedef struct {
 
 std::optional<std::string> GetResponse(std::string url, std::string path);
 std::optional<Update> GetLatestRelease(bool include_prereleases);
+std::optional<Update> GetUpdate();
 } // namespace UpdateChecker