code-style: cleanup includes

* rm unused include <sys/types.h>
* move <sys/time.h> to main.c, it's the only file that needs it.
* move TV_* macros to main.c
* let *.c files explicitly include what they need instead of including
  them at nsxiv.h

image.c

@@ -26,7 +26,6 @@
 #include <stdlib.h>
 #include <string.h>
 #include <sys/stat.h>
-#include <sys/types.h>
 #include <unistd.h>
 
 #if HAVE_LIBEXIF