Add operator= to BigInt

Also make it just be implemented with the default as the tests
show that is enough

Fixes warning in newer gcc

src/botantools/botan/big_base.cpp

@@ -94,6 +94,8 @@ BigInt::BigInt(const BigInt& b)
       }
    }
 
+BigInt &BigInt::operator=(const BigInt&) = default;
+
 /*************************************************
 * Construct a BigInt from a string               *
 *************************************************/

src/botantools/botan/botan/bigint.h

@@ -150,6 +150,7 @@ class BigInt // clazy:exclude=rule-of-three TODO Needs checking if a real bug or
 #ifndef BOTAN_MINIMAL_BIGINT
       BigInt(NumberType, u32bit);
 #endif
+      BigInt &operator=(const BigInt&);
    private:
       void grow_to(u32bit) const;
       SecureVector<word> reg;