Merge bitcoin#31448: fuzz: add cstdlib to FuzzedDataProvider

bb7e686 fuzz: add cstdlib to FuzzedDataProvider (fanquake)

Pull request description:

  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();
  ```

ACKs for top commit:
  dergoegge:
    utACK bb7e686
  brunoerg:
    ACK bb7e686

Tree-SHA512: 22efd5505273ec7254e8dccbb275e648fe02107397c45eff6752e4a6ea787d9d2e45eb0f2ee309df431e9b92ffd14cbcba4b0f4b11a127664466e20be43c383e

Author: fanquake

src/test/fuzz/FuzzedDataProvider.h

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