plug-ins: fix backported DICOM patch.

Commit b44e7daf4c was closing a non-existing `dicom` variable. It was
named `DICOM` in the gimp-2-10 branch.

plug-ins/common/file-dicom.c

@@ -601,7 +601,7 @@ load_image (const gchar  *filename,
                                  "with samples per pixel: %d"),
                                (gchar *) value, samples_per_pixel);
                   g_free (dicominfo);
-                  fclose (dicom);
+                  fclose (DICOM);
                   return NULL;
                 }