libvhost-user: Provide _GNU_SOURCE when compiling outside of QEMU

libvhost-user: Provide _GNU_SOURCE when compiling outside of QEMU

Then the libvhost-user 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       libvhost-user.o
libvhost-user.c: In function ‘vu_panic’:
libvhost-user.c:195:9: error: implicit declaration of function ‘vasprintf’; did you mean ‘vsprintf’? [-Werror=implicit-function-declaration]
  195 |     if (vasprintf(&buf, msg, ap) < 0) {
      |         ^~~~~~~~~
      |         vsprintf

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

Signed-off-by: Marcel Holtmann <marcel@holtmann.org>
Message-Id: <be27dcc747a6b5cc6f8ae3f79e0b79171382bcef.1671741278.git.marcel@holtmann.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>