fix indent of some new priv->modified code

[ploxiln]

Some new code related to priv-modified was added with 4-space indent instead of tab indent, and it triggered warnings in recent GCC for misleading indents. Fix to tab indents (and surrounding brace style).

../src/xviewer-image-jpeg.c:548:5: warning: this ‘if’ clause does not guard... [-Wmisleading-indentation]
  548 |     if (image->priv->modified)
      |     ^~
../src/xviewer-image-jpeg.c:553:9: note: ...this statement, but the latter is misleadingly indented as if it were guarded by the ‘if’
  553 |         if (target == NULL) {
      |         ^~

../src/xviewer-image.c:840:9: warning: this ‘else’ clause does not guard... [-Wmisleading-indentation]
  840 |         else
      |         ^~~~
../src/xviewer-image.c:845:17: note: ...this statement, but the latter is misleadingly indented as if it were guarded by the ‘else’
  845 |                 exif_data_unref (exif);
      |                 ^~~~~~~~~~~~~~~