Fix incorrect return type

fasthash32() calculates a 32-bit hashcode, but the return
type was uint64. Change to uint32.

Noted by Jeff Davis

Discussion: https://postgr.es/m/b16c93e6c736a422d4de668343515375664eb05d.camel%40j-davis.com

src/include/common/hashfn_unstable.h

@@ -361,7 +361,7 @@ fasthash64(const char *k, size_t len, uint64 seed)
 }
 
 /* like fasthash64, but returns a 32-bit hashcode */
-static inline uint64
+static inline uint32
 fasthash32(const char *k, size_t len, uint64 seed)
 {
 	return fasthash_reduce32(fasthash64(k, len, seed));