Uneeded reference to sys/cdefs.h causes musl build to fail

[g4jc]

As title, I was able to build this by removing the uneeded sys/cdefs.h reference. This header is not included in Gentoo musl.

diff --git a/libopenbsd/vis.h b/libopenbsd/vis.h
index abec9c6..c9c89b2 100644
--- a/libopenbsd/vis.h
+++ b/libopenbsd/vis.h
@@ -73,7 +73,7 @@
  */
 #define        UNVIS_END       1       /* no more characters */
 
-#include <sys/cdefs.h>
 
 char   *vis(char *, int, int, int);
 int    strvis(char *, const char *, int);