plug-ins: use only "#?" for RGBE magic number.

It seems that various software use something different after the "#?",
and even Blender code just ended up only use these 2 characters as magic
number. See also bug 792453.

plug-ins/common/file-gegl.c

@@ -73,7 +73,7 @@ static const FileFormat file_formats[] =
     N_("Radiance RGBE"),
     "image/vnd.radiance",
     "hdr",
-    "0,string,?#RADIANCE\x0a",
+    "0,string,?#",
 
     "file-gegl-load-rgbe",
     "Load files in the RGBE file format",