gnatcoll.gpr: fix case to avoid warning value "windows" is not used as label

Change-Id: I68c41db443dc1c813cecf57950e67581a8dadd19

Author: Fabien-Chouteau

gnatcoll.gpr

@@ -266,6 +266,8 @@ project GnatColl is
               use "gnatcoll-os-libc_constants__osx.ads";
             for Implementation ("GNATCOLL.OS.FS.Open_Pipe")
               use "gnatcoll-os-fs-open_pipe__unix.adb";
+         when "windows" =>
+            null;
       end case;
    end Naming;