Eo: Mark class_get() as weak APIs

While eolian-gen was generating EWAPI (weak) class_get()
symbol declarations, they were implemented as EAPI (strong).
Not sure if this mismatch had any significant effect.

This patch tries to address T3423 (windows build).
The mismatch between EAPI and EWAPI might be the problem.

src/lib/eo/Eo.h

@@ -296,7 +296,7 @@ typedef unsigned int Eo_Op;
  * You must use this macro if you want thread safety in class creation.
  */
 #define EO_DEFINE_CLASS(class_get_func_name, class_desc, parent_class, ...) \
-EAPI const Eo_Class * \
+EWAPI const Eo_Class * \
 class_get_func_name(void) \
 { \
    const Eo_Class *_tmp_parent_class; \