Change the type of parameter prevVersion to int in sourceAbiDump

Since the caller is doing Itoa and pass it into sourceAbiDump(), and
sourceAbiDump() is calling Atoi to convert it back, it makes sense to
not do the Itoa in the first place.

Test: make libz
Bug: 244009549
Change-Id: I61731a5eb442b5a1a8f50c820a673d9b2204c3a0