Make HTML Help steps compliant with newer Nmakes.

Nmake 12 seems to be more strict about spaces in commands. Add the
hhc.exe path to PATH and use a bare command instead.

Change-Id: I9fbd9abdabf7f51ac7bac2cbe8d938ae7bb0640e
Reviewed-on: https://code.wireshark.org/review/3192
Reviewed-by: Gerald Combs <gerald@wireshark.org>