preprocessor: fix int/size_t comparison warning for g++

SymbolTable.cc

@@ -110,7 +110,7 @@ SymbolTable::freeze() throw (FrozenException)
 
   frozen = true;
 
-  for (int i = 0; i < symbol_table.size(); i++)
+  for (int i = 0; i < (int) symbol_table.size(); i++)
     {
       int tsi;
       switch (getType(i))

SymbolTable.hh

@@ -367,7 +367,7 @@ public:
 inline void
 SymbolTable::validateSymbID(int symb_id) const throw (UnknownSymbolIDException)
 {
-  if (symb_id < 0 || symb_id > symbol_table.size())
+  if (symb_id < 0 || symb_id > (int) symbol_table.size())
     throw UnknownSymbolIDException(symb_id);
 }