Use the correct conversion specifier for size_t.

Reported by Jim Meyering.
* src/Sbitset.h (SBITSET__INDEX__CONVERSION_SPEC): New, "zu"
because Sbitset__Index is size_t.
* src/Sbitset.c (Sbitset__fprint): Use it instead of %d.
(cherry picked from commit 47eced3099)

src/Sbitset.h

@@ -22,6 +22,7 @@
 
 typedef char *Sbitset;
 typedef size_t Sbitset__Index;
+#define SBITSET__INDEX__CONVERSION_SPEC "zu"
 
 #define Sbitset__nbytes(NBITS)            (((NBITS)+7)/8)
 #define Sbitset__byteAddress(SELF, INDEX) (((SELF) + (INDEX)/8))