build-sys: fix git version from -version

Typo introduced with the script.

Fixes: 2c273f32d3 ("meson: generate qemu-version.h")
Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reported-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Tested-by: Laszlo Ersek <lersek@redhat.com>
Message-Id: <20200929143654.518157-1-marcandre.lureau@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

scripts/qemu-version.sh

@@ -9,7 +9,7 @@ version="$3"
 if [ -z "$pkgversion" ]; then
     cd "$dir"
     if [ -e .git ]; then
-        pkgversion=$(git describe --match 'v*' --dirty | echo "")
+        pkgversion=$(git describe --match 'v*' --dirty) || :
     fi
 fi