ad9361.h: Use a __api tag to mark API functions
ad9361.h: Use a __api tag to mark API functions

This is required for building the library for Windows.

It also allows to reduce the visibility of non-API functions
on other platforms.

Signed-off-by: Paul Cercueil <73d292422978d45c9eeef455bb1b24a0db453722@analog.com>

analogdevicesinc/libad9361-iio,analogdevicesinc/libad9361-iio,analogdevicesinc/libad9361-iio,analogdevicesinc/libad9361-iio