fuzz: don't try and use fopencookie when building for Android

When building for Android, _GNU_SOURCE will be defined, but it doesn't
actually have the fopencookie() function, or define the
cookie_io_functions_t type.

For now just skip trying to use it if we are building for Android.

Should fix bitcoin#22062.

src/test/fuzz/util.h

@@ -384,7 +384,7 @@ class FuzzedFileProvider
             [&] {
                 mode = "a+";
             });
-#ifdef _GNU_SOURCE
+#if defined _GNU_SOURCE && !defined __ANDROID__
         const cookie_io_functions_t io_hooks = {
             FuzzedFileProvider::read,
             FuzzedFileProvider::write,