Effects factory: Restrict visibility to public functions

The next patches will split EffectFactory.c in several files,
thus symbols with internal linkage (static) will be exposed to other
libraries.
To avoid that only export the symbols that are supposed to be public.

Test: Link and boot
Bug: 37492580
Change-Id: Ice2b11b0bc868591a1e17c658ecf26d82cffdaa4
Signed-off-by: Kevin Rocard <krocard@google.com>