Remove X and fontconfig from st.c

None of the X-related includes are needed any longer.  In addition, move
the X modifier defines into x.c, as they are not used outside.

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

st.c

@@ -21,13 +21,8 @@
 #include <time.h>
 #include <unistd.h>
 #include <libgen.h>
-#include <fontconfig/fontconfig.h>
 #include <wchar.h>
 
-/* X11 */
-#include <X11/cursorfont.h>
-#include <X11/Xft/Xft.h>
-
 #include "st.h"
 #include "win.h"