components/*.c: include slstatus.h

this gives the compiler a chance to check weather the prototype and
definiton matches or not, which would catch issues like 3c47701.

components/num_files.c

@@ -4,6 +4,7 @@
 #include <string.h>
 
 #include "../util.h"
+#include "../slstatus.h"
 
 const char *
 num_files(const char *path)