[introspection] Fix g-ir-scanner syntax errors

Fixes https://github.com/harfbuzz/harfbuzz/issues/2851
diff --git a/src/hb-ot-font.h b/src/hb-ot-font.h
index 80eaa54..e7959d1 100644
--- a/src/hb-ot-font.h
+++ b/src/hb-ot-font.h
@@ -24,7 +24,7 @@
  * Google Author(s): Behdad Esfahbod, Roozbeh Pournader
  */
 
-#ifndef HB_OT_H_IN
+#if !defined(HB_OT_H_IN) && !defined(HB_NO_SINGLE_HEADER_ERROR)
 #error "Include <hb-ot.h> instead."
 #endif