text: include errno.h in text.h

text.h uses -EOPNOTSUPP so we should include errno.h to avoid compilation
errors.

Signed-off-by: David Herrmann <dh.herrmann@googlemail.com>

src/text.h

@@ -38,6 +38,7 @@
 #ifndef KMSCON_TEXT_H
 #define KMSCON_TEXT_H
 
+#include <errno.h>
 #include <stdlib.h>
 #include "unicode.h"
 #include "uterm.h"