osmo_io: Dont use __linux__ but !EMBEDDED

osmo_io (unlike its io_uring backend) is not linux specific, so putting
it in an "#ifdef __linux__" block is plain wrong.

A side-effect is that all our doxygen comments are not processed as
__linux__ is not set while doxygen parses the source file.

Change-Id: I260443ba3cd0bb81dade434208dea4ea70fe8ad8
diff --git a/src/core/osmo_io.c b/src/core/osmo_io.c
index 57eba6c..745d783 100644
--- a/src/core/osmo_io.c
+++ b/src/core/osmo_io.c
@@ -21,7 +21,7 @@
  */
 
 #include "../config.h"
-#if defined(__linux__)
+#ifndef EMBEDDED
 
 #include <fcntl.h>
 #include <stdio.h>
@@ -934,4 +934,4 @@
 }
 
 
-#endif /* defined(__linux__) */
+#endif /* ifndef(EMBEDDED) */