Since commit f5f169ba99 "Split build rules for tools to allow
skipping them." debian/rules.d/Makefile is not used and the current
kernel's UAPI headers are not installed. This hasn't caused breakage
yet, probably because many tools have their own workaround using
include/uapi etc. directly, but could break backports builds at some
point.
Move the build of userland headers up into debian/rules.real and
make all tools build targets depend on it.