Make win variable internal to x.c

There was only a single reference to the `win` variable in st.c, so
exporting that to x.c allows us to rid ourselves of another extern.

Signed-off-by: Devin J. Pohly <djpohly@gmail.com>

st.h

@@ -201,7 +201,6 @@ void *xrealloc(void *, size_t);
 char *xstrdup(char *);
 
 /* Globals */
-extern TermWindow win;
 extern Term term;
 extern Selection sel;
 extern int cmdfd;