Remove warnings when building without EFL_BETA_API_SUPPORT
efl_alive_get should be a method defined in efl_object.eo, protected by
EFL_BETA_API_SUPPORT while that class is still @beta.
Instead, it is defined in Eo.h, so the BETA guard has to be defined manually.
This was causing some unsighly warnings during "make" and "make check"
(implicit declaration of function ‘efl_invalidating_get’)
Test Plan: make && make check do not throw that warning anymore
Reviewers: zmike, cedric
Reviewed By: zmike
Differential Revision: https://phab.enlightenment.org/D7958