meson: Only take the version from dtc's own git repository

When dtc is built from a release tarball unpacked inside another git
repository, git describe finds that repository and its tag ends up in the
version string, so dtc --version reports the tag rather than "DTC 1.8.1".
Point git at dtc's own .git, so that vcs_tag falls back to the project
version when there is none.

Signed-off-by: Rudi Heitbaum <rudi@heitbaum.com>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>

meson.build

diff --git a/meson.build b/meson.build
index 501b706..520cd35 100644
--- a/meson.build
+++ b/meson.build
@@ -47,7 +47,7 @@
 wheel_only = get_option('wheel-only')
 
 version_gen_h = vcs_tag(
-  command: ['git', 'describe', '--dirty=+'],
+  command: ['git', '--git-dir=.git', 'describe', '--dirty=+'],
   input: 'version_gen.h.in',
   output: 'version_gen.h',
 )