restore the declaration of _upb_Hash()

It turns out that this function is, annoyingly, being used by Ruby.

PiperOrigin-RevId: 490305008

upb/hash/common.h

@@ -185,8 +185,11 @@ UPB_INLINE size_t upb_table_size(const upb_table* t) {
 }
 
 // Internal-only functions, in .h file only out of necessity.
+
 UPB_INLINE bool upb_tabent_isempty(const upb_tabent* e) { return e->key == 0; }
 
+uint32_t _upb_Hash(const void* p, size_t n, uint64_t seed);
+
 #ifdef __cplusplus
 } /* extern "C" */
 #endif