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/load_avg.c

@@ -3,6 +3,7 @@
 #include <stdlib.h>
 
 #include "../util.h"
+#include "../slstatus.h"
 
 const char *
 load_avg(void)