Fix wvstreams so that it builds with uClibc: const cast problem.

Signed-off-by: Simon Dawson <spdawson@gmail.com>

diff -Nurp a/crypto/wvx509.cc b/crypto/wvx509.cc
--- a/crypto/wvx509.cc	2008-10-23 21:23:49.000000000 +0100
+++ b/crypto/wvx509.cc	2012-06-15 18:45:06.605899292 +0100
@@ -1157,7 +1157,7 @@ WvString WvX509::get_extension(int nid)
         
         if (ext)
         {
-            X509V3_EXT_METHOD *method = X509V3_EXT_get(ext);
+          X509V3_EXT_METHOD *method = const_cast<X509V3_EXT_METHOD *>(X509V3_EXT_get(ext));
             if (!method)
             {
                 WvDynBuf buf;