Check in .clang-ignore

The CI format-code.sh script uses a .clang-ignore to determine which
libraries are vendored.  Add a script for bmcweb that allows skipping
our subprojects directory.

Tested:
No code changed.  Devtool only.

Signed-off-by: Ed Tanous <ed@tanous.net>
Change-Id: Ice67a678c93062c2ae1fa11719d4372117df6e46
diff --git a/.clang-ignore b/.clang-ignore
new file mode 100644
index 0000000..36f17f2
--- /dev/null
+++ b/.clang-ignore
@@ -0,0 +1 @@
+./subprojects