Verify that header sections are found even if not used

This prevents typos in the endif part of the section even when test
coverage is not perfect.
diff --git a/gen/write.rs b/gen/write.rs
index d80e957..4eed1b4 100644
--- a/gen/write.rs
+++ b/gen/write.rs
@@ -261,9 +261,10 @@
 }
 
 fn write_header_section(out: &mut OutFile, needed: bool, section: &str) {
+    let section = include::get(section);
     if needed {
         out.next_section();
-        for line in include::get(section).lines() {
+        for line in section.lines() {
             if !line.trim_start().starts_with("//") {
                 writeln!(out, "{}", line);
             }