fuzz: add cstdlib to FuzzedDataProvider

Same as llvm/llvm-project#113951.

Avoids compile failures under clang-20 &
`D_LIBCPP_REMOVE_TRANSITIVE_INCLUDES`:
```bash
In file included from /bitcoin/src/test/fuzz/addition_overflow.cpp:5:
/bitcoin/src/test/fuzz/FuzzedDataProvider.h:209:5: error: use of undeclared identifier 'abort'
  209 |     abort();
      |     ^
/bitcoin/src/test/fuzz/FuzzedDataProvider.h:250:5: error: use of undeclared identifier 'abort'
  250 |     abort();
```

src/test/fuzz/FuzzedDataProvider.h

@@ -18,6 +18,7 @@
 #include <climits>
 #include <cstddef>
 #include <cstdint>
+#include <cstdlib>
 #include <cstring>
 #include <initializer_list>
 #include <limits>