lib: work around forward reference to enum types

lib: work around forward reference to enum types

Exported header files used by applications should allow the strictest
compiler flags. Language extensions used in many places must be explicitly
marked or removed to avoid warnings and compilation failures.

This commit prevents the following errors:

 error: ISO C forbids forward references to `enum' types

Signed-off-by: Adrien Mazarguil <adrien.mazarguil@6wind.com>