Minor cleanup of fuzz_target.cc

- Include What You Use (IWYU) - add cstddef
- Convert to C++ includes of C headers
- Localize vars
- Combine definition and initialization of dest
- Convert C cast to C++ cast
- Remove unused source var

fuzzing/fuzz_target.cc

-#include <stdlib.h>
-#include <stdint.h>
+#include <cstddef>
+#include <cstdint>
+#include <cstdlib>
 #include "libaec.h"
 
 extern "C" int LLVMFuzzerTestOneInput(const uint8_t *Data, size_t Size) {
-    struct aec_stream strm;
-    int32_t *source;
-    unsigned char *dest;
-
     if (Size < 2)
         return 0;
 
-    dest = (unsigned char *)malloc(Size * 4);
+    unsigned char *dest = static_cast<unsigned char *>(malloc(Size * 4));
+    struct aec_stream strm;
     strm.bits_per_sample = (Data[0] & 0x1f) | 1;
     strm.block_size = 8 << (Data[1] & 3);
     strm.rsi = 2;