Move forward declaration of class (to help doxygen)

Before this commit, get_xml_options had the parameters
"(const class xmlt &xml, cmdlinet &cmdline)" in the header file, and
"(const xmlt &xml, cmdlinet &cmdline)" in the cpp file.  Doxygen then
produced a warning "no matching class member found".  This commit
adds a forward declaration "class xmlt;" in the header file, so that
both versions of get_xml_options can have identical parameter lists.

Author: John Nonweiler

src/cbmc/xml_interface.h

@@ -14,6 +14,8 @@ Author: Daniel Kroening, [email protected]
 
 #include <util/cmdline.h>
 
+class xmlt;
+
 class xml_interfacet
 {
 public:
@@ -24,7 +26,7 @@ class xml_interfacet
 
 protected:
   void get_xml_options(cmdlinet &cmdline);
-  void get_xml_options(const class xmlt &xml, cmdlinet &cmdline);
+  void get_xml_options(const xmlt &xml, cmdlinet &cmdline);
 };
 
 #endif // CPROVER_CBMC_XML_INTERFACE_H