RFR(XS): 8218544: Fix build on windows: missing include

[Lindenmaier, Goetz]

Hi

the windows build is broken for a few days. This adds the missing
include.
http://cr.openjdk.java.net/~goetz/wr19/8218544-win_build/01/

Please review.

disassembler.obj : error LNK2019: unresolved external symbol "public: static char const * __cdecl os::dll_file_extension(void)"

Best regards,
  Goetz.