msvc.mak: Update insns.pl arguments

45a22d9a6147c320b6754679de54a8290de3c5e3 changes the arguments of insns.pl
and adds a new autogenerated header iflaggen.h. Update the nmake Makefile
accordingly.

Signed-off-by: Jim Kukunas <james.t.kukunas@linux.intel.com>