fuzz: restrict fopencookie usage to Linux & FreeBSD

Should fix the GCC compilation portion of bitcoin#29517:
bitcoin#29517 (comment).

See also:
https://www.gnu.org/software/gnulib/manual/html_node/fopencookie.html
but note that FreeBSD has supported this function since 11.x.

src/test/fuzz/util.cpp

@@ -272,7 +272,7 @@ FILE* FuzzedFileProvider::open()
         [&] {
             mode = "a+";
         });
-#if defined _GNU_SOURCE && !defined __ANDROID__
+#if defined _GNU_SOURCE && (defined(__linux__) || defined(__FreeBSD__))
     const cookie_io_functions_t io_hooks = {
         FuzzedFileProvider::read,
         FuzzedFileProvider::write,