libvduse: Provide _GNU_SOURCE when compiling outside of QEMU

libvduse: Provide _GNU_SOURCE when compiling outside of QEMU

When the libvduse sources are used by another project, it can not be
guaranteed that _GNU_SOURCE is set by the build system. If it is for
example not set, errors like this show up.

  CC       libvduse.o
libvduse.c: In function ‘vduse_log_get’:
libvduse.c:172:9: error: implicit declaration of function ‘ftruncate’; did you mean ‘strncat’? [-Werror=implicit-function-declaration]
  172 |     if (ftruncate(fd, size) == -1) {
      |         ^~~~~~~~~
      |         strncat

The simplest way to allow external complication of libvduse.[ch] by
setting _GNU_SOURCE if it is not already set by the build system.

Signed-off-by: Marcel Holtmann <marcel@holtmann.org>
Message-Id: <407f3665f0605df936e5bfe60831d180edfb8cca.1671741278.git.marcel@holtmann.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>