Add missing prototype for p_realpath().
diff --git a/src/unix/posix.h b/src/unix/posix.h
index f4886c5..9c9f837 100644
--- a/src/unix/posix.h
+++ b/src/unix/posix.h
@@ -21,6 +21,8 @@
/* The OpenBSD realpath function behaves differently */
#if !defined(__OpenBSD__)
# define p_realpath(p, po) realpath(p, po)
+#else
+char *p_realpath(const char *, char *);
#endif
#define p_vsnprintf(b, c, f, a) vsnprintf(b, c, f, a)