rfb: Fix hidden function overload warnings

Fix the following warnings emitted by Clang:

/home/shade/dev/tigervnc/common/rfb/Configuration.h:164:18: note: hidden overloaded virtual function 'rfb::VoidParameter::setParam' declared here: different number of parameters (0 vs 1)
    virtual bool setParam();
                 ^
/home/shade/dev/tigervnc/common/rfb/Configuration.h:256:18: error: 'rfb::BinaryParameter::setParam' hides overloaded virtual function [-Werror,-Woverloaded-virtual]
    virtual void setParam(const void* v, int l);
                 ^
/home/shade/dev/tigervnc/common/rfb/Configuration.h:164:18: note: hidden overloaded virtual function 'rfb::VoidParameter::setParam' declared here: different number of parameters (0 vs 2)
    virtual bool setParam();
diff --git a/common/rfb/Configuration.h b/common/rfb/Configuration.h
index fbf161d..d319915 100644
--- a/common/rfb/Configuration.h
+++ b/common/rfb/Configuration.h
@@ -215,6 +215,7 @@
     IntParameter(const char* name_, const char* desc_, int v,
                  int minValue=INT_MIN, int maxValue=INT_MAX,
 		 ConfigurationObject co=ConfGlobal);
+    using VoidParameter::setParam;
     virtual bool setParam(const char* value);
     virtual bool setParam(int v);
     virtual char* getDefaultStr() const;
@@ -251,6 +252,7 @@
   public:
     BinaryParameter(const char* name_, const char* desc_, const void* v, int l,
 		    ConfigurationObject co=ConfGlobal);
+    using VoidParameter::setParam;
     virtual ~BinaryParameter();
     virtual bool setParam(const char* value);
     virtual void setParam(const void* v, int l);