compat: Use absl::string_view instead of google::protobuf::internal::DescriptorStringView

Even though google::protobuf::internal::DescriptorStringView is exposed
in public protobuf headers, it's probably not a good idea to rely on an
"internal" typedef.

According to https://protobuf.dev/news/2024-10-02/#descriptor-apis:

    v30 will update return types in descriptor (such as full_name) to be
    absl::string_view.

So `absl::string_view` is probably the right type to use here.

protoc-gen-c/compat.h

@@ -37,12 +37,16 @@
 # define GOOGLE_LOG		ABSL_LOG
 #endif
 
+#if GOOGLE_PROTOBUF_VERSION >= 6030000
+# include <absl/strings/string_view.h>
+#endif
+
 namespace protobuf_c {
 
 namespace compat {
 
 #if GOOGLE_PROTOBUF_VERSION >= 6030000
-typedef google::protobuf::internal::DescriptorStringView StringView;
+typedef absl::string_view StringView;
 #else
 typedef const std::string& StringView;
 #endif