Added \platform annotations.
diff --git a/Doc/lib/libgdbm.tex b/Doc/lib/libgdbm.tex
index e787454..585693c 100644
--- a/Doc/lib/libgdbm.tex
+++ b/Doc/lib/libgdbm.tex
@@ -2,6 +2,7 @@
          GNU's reinterpretation of dbm}
 
 \declaremodule{builtin}{gdbm}
+  \platform{UNIX}
 \modulesynopsis{GNU's reinterpretation of dbm.}