Merge bitcoin/bitcoin#28506: fuzz: Add missing PROVIDE_FUZZ_MAIN_FUNCTION guard to __AFL_FUZZ_INIT

fa33b2c889 fuzz: Add missing PROVIDE_FUZZ_MAIN_FUNCTION guard to __AFL_FUZZ_INIT (MarcoFalke)

Pull request description:

  Should fix https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=62455

ACKs for top commit:
  dergoegge:
    utACK fa33b2c889

Tree-SHA512: 735926f7f94ad1c3c5dc0fc62a2ef3a85abae25f4fe1e7654c2857ce3e867667ed28da58ab36281d730d3e206a0728cb429671ea5d3ccd11519e637eb191f70d

src/test/fuzz/fuzz.cpp

@@ -29,7 +29,7 @@
 #include <utility>
 #include <vector>
 
-#ifdef __AFL_FUZZ_INIT
+#if defined(PROVIDE_FUZZ_MAIN_FUNCTION) && defined(__AFL_FUZZ_INIT)
 __AFL_FUZZ_INIT();
 #endif