glibc: fix glibc build by creating an empty gnu/stubs.h

The stubs.h header is not installed by install-headers, but is needed
for the gcc build. An empty stubs.h will work, as explained in
http://gcc.gnu.org/ml/gcc/2002-01/msg00900.html

. The same trick is
used by Crosstool-NG.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>