ggml-backend : unify the dl_load_library() return type

Use the unified type 'dl_handle' instead of 'void' directly, to match
the Windows 'dl_load_library()' definition.

Author: haiyuewa

ggml/src/ggml-backend-reg.cpp

@@ -149,7 +149,7 @@ struct dl_handle_deleter {
     }
 };
 
-static void * dl_load_library(const fs::path & path) {
+static dl_handle * dl_load_library(const fs::path & path) {
     dl_handle * handle = dlopen(path.string().c_str(), RTLD_NOW | RTLD_LOCAL);
 
     return handle;