adt/common: remove const from return type

reported by LGTM https://lgtm.com/projects/g/srsLTE/srsLTE/rev/pr-9ae46cae260b6b8137b34b9423bf1477bd6b5c42

lib/include/srsran/adt/interval.h

@@ -39,7 +39,7 @@ public:
 
   bool empty() const { return stop_ == start_; }
 
-  auto length() -> decltype(std::declval<T>() - std::declval<T>()) const { return stop_ - start_; }
+  auto length() const -> decltype(std::declval<T>() - std::declval<T>()) { return stop_ - start_; }
 
   void set(T start_point, T stop_point)
   {

lib/include/srsran/common/byte_buffer.h

@@ -155,7 +155,7 @@ public:
   const uint8_t* data() const { return msg; }
   uint32_t       size() const { return N_bytes; }
   iterator       begin() { return msg; }
-  const iterator begin() const { return msg; }
+  const_iterator begin() const { return msg; }
   iterator       end() { return msg + N_bytes; }
   const_iterator end() const { return msg + N_bytes; }