fix: sidestep -Wunused-function warnings with [[maybe_unused]]

Code is unused but inherited by upstream and therefore, has to stick
around even if we do nothing with it.

Author: kwvg

src/script/bitcoinconsensus.cpp

@@ -48,7 +48,7 @@ class TxInputStream
         return *this;
     }
 
-    int GetVersion() const { return m_version; }
+    [[maybe_unused]] int GetVersion() const { return m_version; }
 private:
     const int m_version;
     const unsigned char* m_data;

src/script/interpreter.cpp

@@ -1521,7 +1521,7 @@ template void PrecomputedTransactionData::Init(const CMutableTransaction& txTo,
 template PrecomputedTransactionData::PrecomputedTransactionData(const CTransaction& txTo);
 template PrecomputedTransactionData::PrecomputedTransactionData(const CMutableTransaction& txTo);
 
-static bool HandleMissingData(MissingDataBehavior mdb)
+[[maybe_unused]] static bool HandleMissingData(MissingDataBehavior mdb)
 {
     switch (mdb) {
     case MissingDataBehavior::ASSERT_FAIL:

src/test/script_tests.cpp

@@ -282,19 +282,19 @@ class TestBuilder
         return *this;
     }
 
-    TestBuilder& Push(const std::string& hex)
+    [[maybe_unused]] TestBuilder& Push(const std::string& hex)
     {
         DoPush(ParseHex(hex));
         return *this;
     }
 
-    TestBuilder& Push(const uint256& hash)
+    [[maybe_unused]] TestBuilder& Push(const uint256& hash)
     {
         DoPush(ToByteVector(hash));
         return *this;
     }
 
-    TestBuilder& Push(const CScript& script)
+    [[maybe_unused]] TestBuilder& Push(const CScript& script)
     {
          DoPush(std::vector<unsigned char>(script.begin(), script.end()));
         return *this;