eina: fix define of Eina_Content

there is no need to declare that const.

src/lib/eina/eina_abstract_content.c

@@ -184,7 +184,7 @@ eina_content_type_get(Eina_Content *content)
    return content->type;
 }
 
-EAPI const Eina_Slice
+EAPI Eina_Slice
 eina_content_data_get(Eina_Content *content)
 {
    return eina_rw_slice_slice_get(content->data);

src/lib/eina/eina_abstract_content.h

@@ -63,7 +63,7 @@ EAPI const char* eina_content_type_get(Eina_Content *content);
  *
  * @return An Eina_Slice containing the data. Do not free.
  */
-EAPI const Eina_Slice eina_content_data_get(Eina_Content *content);
+EAPI Eina_Slice eina_content_data_get(Eina_Content *content);
 
 /**
  * Create a new content object, with the provided data and type.