platform: zephyr: add fs.h include

[martinjaeger]

Required for definition of struct fs_file_t and struct fs_dir_t.

Without this fix I get the following build error (when building for Zephyr):

.../wasm-micro-runtime/core/shared/platform/zephyr/platform_internal.h:269:26: error: field 'file' has incomplete type
  269 |         struct fs_file_t file;
      |                          ^~~~
.../wasm-micro-runtime/core/shared/platform/zephyr/platform_internal.h:270:25: error: field 'dir' has incomplete type
  270 |         struct fs_dir_t dir;