Add reallocarray prototype.

compat.h

@@ -264,6 +264,11 @@ void		cfmakeraw(struct termios *);
 int		 openat(int, const char *, int, ...);
 #endif
 
+#ifndef HAVE_REALLOCARRAY
+/* reallocarray.c */
+void		*reallocarray(void *, size_t, size_t size);
+#endif
+
 #ifdef HAVE_GETOPT
 #include <getopt.h>
 #else