Fix build with gcc6

Downloaded from
https://build.opensuse.org/package/view_file/devel:libraries:c_c++/xmlrpc-c/narrowing.patch

Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>

Index: xmlrpc-c-1.33.18/src/cpp/base64.cpp
===================================================================
--- xmlrpc-c-1.33.18.orig/src/cpp/base64.cpp
+++ xmlrpc-c-1.33.18/src/cpp/base64.cpp
@@ -14,7 +14,7 @@ using namespace xmlrpc_c;
 
 namespace {
 
-char const table_a2b_base64[] = {
+signed char const table_a2b_base64[] = {
     -1,-1,-1,-1, -1,-1,-1,-1, -1,-1,-1,-1, -1,-1,-1,-1,
     -1,-1,-1,-1, -1,-1,-1,-1, -1,-1,-1,-1, -1,-1,-1,-1,
     -1,-1,-1,-1, -1,-1,-1,-1, -1,-1,-1,62, -1,-1,-1,63,