shell.c: Include sys/wait.h

Required for the W* macros on OpenBSD. Reported by Brian Callahan.

src/shell.c

@@ -32,6 +32,7 @@
 #include <ncurses.h>
 #include <stdlib.h>
 #include <unistd.h>
+#include <sys/wait.h>
 
 void shell_draw() {
   char *full_path;