fdio: Be const-correct for struct stat

We don't modify this struct (if non-NULL), so it can be const. In
particular, this is helpful if calling glnx_file_copy_at() from
nftw() to implement the equivalent of `cp -a --reflink=auto`.

Signed-off-by: Simon McVittie <smcv@collabora.com>

glnx-fdio.c

@@ -939,7 +939,7 @@ glnx_regfile_copy_bytes (int fdf, int fdt, off_t max_bytes)
 gboolean
 glnx_file_copy_at (int                   src_dfd,
                    const char           *src_subpath,
-                   struct stat          *src_stbuf,
+                   const struct stat    *src_stbuf,
                    int                   dest_dfd,
                    const char           *dest_subpath,
                    GLnxFileCopyFlags     copyflags,

glnx-fdio.h

@@ -193,7 +193,7 @@ typedef enum {
 gboolean
 glnx_file_copy_at (int                   src_dfd,
                    const char           *src_subpath,
-                   struct stat          *src_stbuf,
+                   const struct stat    *src_stbuf,
                    int                   dest_dfd,
                    const char           *dest_subpath,
                    GLnxFileCopyFlags     copyflags,