Add libintl.h to the experimental API.

git-svn-id: https://svn.r-project.org/R/trunk@89088 00db46b3-68df-0310-9c12-caf00c1e9a41

Author: luke

doc/manual/R-exts.texi

@@ -6926,6 +6926,7 @@ C/C++) files containing messages that should be translated, declare
 #define _(String) (String)
 #endif
 @end example
+@eapihdr libintl.h
 
 @item
 For each message that should be translated, wrap it in @code{_(...)},