Make INPUT_FSTAT_NB_READS a power of two to ease division

Some instruction sets can't divide (e.g. ARM), but pretty much all of
them can shift bits.

include/vlc_config.h

@@ -74,7 +74,7 @@
 
 /* Number of read() calls needed until we check the file size through
  * fstat() */
-#define INPUT_FSTAT_NB_READS            10
+#define INPUT_FSTAT_NB_READS            16
 
 /*
  * General limitations