Add checks for HAVE_GPL to various GPL-only source files

This should actually cover all of them, if you take into account that
some unchanged GPL source files include header files with such checks.
Also this was done already for the libaf derived code.

This is only for "safety" and to avoid misunderstandings.

stream/stream_cdda.c

@@ -46,6 +46,11 @@
 
 #include "common/msg.h"
 
+#include "config.h"
+#if !HAVE_GPL
+#error GPL only
+#endif
+
 typedef struct cdda_params {
     cdrom_drive_t *cd;
     cdrom_paranoia_t *cdp;