Remove apostrophes from instances of possessive "its".

I noticed one instance of this while looking at the code the other day,
and after a quick grep, realized it happened a *lot*!  One of the many
frustrating things about the English language is that we use apostrophes
to make pretty much everything possessive *except* the pronoun "it".
In that case, we use "its".  "It's" is reserved for the contraction
meaning "it is" or "it has".

src/window.h

@@ -173,7 +173,7 @@ typedef struct WFakeGroupLeader {
 
 
 /*
- * Stores client window information. Each client window has it's
+ * Stores client window information. Each client window has its
  * structure created when it's being first mapped.
  */
 typedef struct WWindow {