The prototype means "unspecified arguments". However, the implementation
specifies no parameters (void).
GCC's warning -Wstrict-prototypes complains a lots about this old-style
prototype, spamming the console when compiling more than one compiling
units including Eina.
@fix
Eina Value Optional can be used to create a eina value that can be set
or be empty and can be embedded in a eina_value_struct.
Signed-off-by: Cedric BAIL <cedric@osg.samsung.com>