Makes winbindd_proto handling consistent with the other header files,

Makes winbindd_proto handling consistent with the other header files,
so that at least "make clean; make all" doesn't fail.

It's still not safe for parallel builds, i.e. "make clean; make -j3
all" will probably crash, but "make clean; make proto && make -j3 all"
seems OK.  I'm not sure if it is possible to solve that and still
support ancient non-GNU versions of Make.
(This used to be commit 4d202c19997d4219e7f607a725123d5606b9bd8a)