Declare set_file_content_provider for the split build

FileContentProviderStaysWithinFile, added in e374966, calls
detail::set_file_content_provider, which split.py moves into httplib.cc,
so test_split did not compile. Declare it in the test, as the other tests
that call into detail:: already do.

test/test.cc

@@ -12354,6 +12354,15 @@ TEST(ResponseContentSourceTest, LastContentSetterWins) {
   }
 }
 
+// Forward declaration (see the base64_encode note).
+namespace httplib {
+namespace detail {
+void set_file_content_provider(Response &res, const std::shared_ptr<mmap> &m,
+                               const std::string &content_type,
+                               EncodingType encoding);
+} // namespace detail
+} // namespace httplib
+
 TEST(ResponseContentSourceTest, FileContentProviderStaysWithinFile) {
   auto mm = std::make_shared<detail::mmap>("./www/dir/test.html");
   ASSERT_TRUE(mm->is_open());