classads: add a missing dependency

pkgs/development/libraries/classads/default.nix

@@ -1,4 +1,4 @@
-{ stdenv, fetchurl }:
+{ stdenv, fetchurl, pcre }:
 
 let version = "1.0.10"; in
 
@@ -10,6 +10,8 @@ stdenv.mkDerivation {
     sha256 = "1czgj53gnfkq3ncwlsrwnr4y91wgz35sbicgkp4npfrajqizxqnd";
   };
 
+  buildInputs = [ pcre ];
+
   configureFlags = ''                                                  
     --enable-namespace --enable-flexible-member
   '';