Update fuzz_manifest.c: Add missing include unistd.h

[maflcko]

To fix a clang-18 error:

test/fuzzing/fuzz_manifest.c:25:5: error: call to undeclared function 'unlink'; ISO C99 and later do not support implicit function declarations [-Wimplicit-function-declaration]
   25 |     unlink(filename);
      |     ^
1 error generated.
ERROR:__main__:Building fuzzers failed.