Fix dropbear compilation with glibc

The patch introduced at c579e32d was
incorrect, since __USE_GNU is an internal C library defined, and
should not be used by programs. _GNU_SOURCE should be used instead.

This fixes a build failure on Dropbear when compiling against a
glibc-based toolchain.

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