Add != operator to symbolt

This enables code which uses `symbolt` to check whether two symbols are
different using `symbol_a != symbol_b` instead of `!(symbol_a ==
symbol_b)`.

Author: thomasspriggs

src/util/symbol.cpp

@@ -176,3 +176,8 @@ bool symbolt::operator==(const symbolt &other) const
     is_volatile == other.is_volatile;
   // clang-format on
 }
+
+bool symbolt::operator!=(const symbolt &other) const
+{
+  return !(*this == other);
+}

src/util/symbol.h

@@ -126,6 +126,7 @@ class symbolt
   bool is_well_formed() const;
 
   bool operator==(const symbolt &other) const;
+  bool operator!=(const symbolt &other) const;
 };
 
 std::ostream &operator<<(std::ostream &out, const symbolt &symbol);