fix one of the Debian patches -- it uses warnx but doesn't include err.h.

--- a/src/eval.c
+++ b/src/eval.c
@@ -36,6 +36,7 @@
 #include <signal.h>
 #include <unistd.h>
 #include <sys/types.h>
+#include <err.h>
 
 /*
  * Evaluate a command.