vfs: define retro_vfs_file_punch_hole_impl on the UWP backend

Fixes the parity failure my previous commit caused. The UWP backend
replaces vfs_implementation.c rather than extending it, so a function
added to the C backend and called from the shared file_stream.c
compiles everywhere and fails to link on UWP alone.

tools/vfs_backend_parity.py exists precisely because
retro_vfs_file_get_mapped_ptr_impl once shipped that way, and it caught
this within a minute of the push. I did not run it, or
libretro-common/Makefile.test, before sending the patch -- I verified
the behaviour of the new call and the build of the two trees I had
locally, and never looked for the repository's own checks. That is the
actual mistake here; the missing stub is just its symptom.

UWP cannot punch holes: FSCTL_SET_ZERO_DATA goes through
DeviceIoControl, which is not in the app container's API surface, and
this backend's handle comes from a StorageFile. So it returns the
documented -1, which callers of filestream_punch_hole already handle by
writing zeroes instead -- the space saving is lost, nothing else.

Checked with the tools the CI uses: vfs_backend_parity.py reports 23
symbols with both backends agreeing, and libretro-common/Makefile.test
passes including the Windows A/W configuration matrix.

libretro-common/vfs/vfs_implementation_uwp.cpp

@@ -672,6 +672,25 @@ const char *retro_vfs_file_get_path_impl(libretro_vfs_implementation_file *strea
    return stream->orig_path;
 }
 
+int retro_vfs_file_punch_hole_impl(libretro_vfs_implementation_file *stream,
+      int64_t offset, int64_t len)
+{
+   /* UWP cannot punch holes. FSCTL_SET_ZERO_DATA is issued through
+    * DeviceIoControl, which is not in the app container's API surface,
+    * and this backend's handle comes from a StorageFile rather than
+    * being one a filesystem control code can be sent to.
+    *
+    * -1 is the documented "not available" answer: filestream_punch_hole
+    * is a capability, so callers write zeroes instead and lose the space
+    * saving and nothing else. Defined here rather than omitted because
+    * this backend REPLACES vfs_implementation.c on UWP, so a missing
+    * definition is a link error at the end of a long MSVC build. */
+   (void)stream;
+   (void)offset;
+   (void)len;
+   return -1;
+}
+
 const uint8_t *retro_vfs_file_get_mapped_ptr_impl(
       libretro_vfs_implementation_file *stream, int64_t *len)
 {