Don't export iio_context_create_from_backend as API function

As far as I know it is not used anywhere outside libiio; so keep it
private a bit longer until we have a plug-in system that's robust
enough to have actual implementations.

Signed-off-by: Paul Cercueil <[email protected]>

Author: pcercuei

iio-backend.h

@@ -84,7 +84,7 @@ struct iio_backend {
 	unsigned int			sizeof_context_pdata;
 };
 
-__api struct iio_context * iio_context_create_from_backend(
+struct iio_context * iio_context_create_from_backend(
 		const struct iio_backend *backend,
 		const char *description);