Formatting commit :(

I know formatting commits suck... And I try to avoid them.

But this commit was absolutely necessary... The coding style in this
program was not ok and not the same over the whole program.

The commit is hard to read, but this is what I changed:
- Tabs for indentation instead of spaces
- Same style over the whole program (suckless style)

slstatus.h

@@ -6,9 +6,9 @@ static Display *dpy;
 /* statusbar configuration type and struct */
 typedef char *(*op_fun) (const char *);
 struct arg {
-    op_fun func;
-    const char *format;
-    const char *args;
+	op_fun func;
+	const char *format;
+	const char *args;
 };
 
 /* functions */