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>

win.h

@@ -14,5 +14,6 @@ void xhints(void);
 void xloadcols(void);
 int xsetcolorname(int, const char *);
 void xsettitle(char *);
+int xsetcursor(int);
 void xsetpointermotion(int);
 void xsetsel(char *, Time);