Made the overlay demo programs compile for CBM targets again.

<conio.h> includes target-specific headers; so, we didn't bother to include <cbm.h> where it is needed.  But, '#include <conio.h>' was removed from some files; so now, we must include <cbm.h> explicitly.

samples/multidemo.c

@@ -20,6 +20,7 @@
 #include <fcntl.h>
 #include <unistd.h>
 #else
+#include <cbm.h>
 #include <device.h>
 #endif
 

samples/overlaydemo.c

@@ -15,6 +15,7 @@
 #include <fcntl.h>
 #include <unistd.h>
 #else
+#include <cbm.h>
 #include <device.h>
 #endif