Fix dropbear compilation with glibc

The patch introduced at c579e32dbe6d927bdcfffeede18123aa92abbdd5 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>

package/dropbear/dropbear-fix-missing-TIMEVAL_TO_TIMESPEC.patch

@@ -5,7 +5,7 @@ diff -ur dropbear-0.48/includes.h dropbear-0.48-patched/includes.h
  #include <sys/param.h> /* required for BSD4_4 define */
  #include <sys/socket.h>
  #include <sys/stat.h>
-+#define __USE_GNU
++#define _GNU_SOURCE
  #include <sys/time.h>
  #include <sys/un.h>
  #include <sys/wait.h>