do not ignore build failures in subdirs of iproute2

do not ignore build failures in subdirs of iproute2

if a file fails to compile in a subdir of iproute2 fails (say "tunnel.c" in
the "ip" dir), the top level makefile does not abort:
 all: Config
    @for i in $(SUBDIRS); \
    do $(MAKE) $(MFLAGS) -C $$i; done

the attached patch inserts a 'set -e' so that if the $(MAKE) fails, the all
target fails as well
-mike