Update list of platforms that have a list of exported symbols.

src/port/README

@@ -23,4 +23,6 @@ applications.
 from libpgport are linked first.  This avoids having applications
 dependent on symbols that are _used_ by libpq, but not intended to be
 exported by libpq.  libpq's libpgport usage changes over time, so such a
-dependency is a problem.  Win32 uses export list of symbols for libpq.
+dependency is a problem.  Win32, Linux, and Darin use an export list to
+control the symbols exported by libpq.
+