update clang-ignore

Don't run clang-format on (meson) subprojects.

Change-Id: Ic48ed43d51ccdff3e32a1bf981c61d591dc15e04
Signed-off-by: Brad Bishop <bradleyb@fuzziesquirrel.com>
diff --git a/.clang-ignore b/.clang-ignore
index 9376935..978b461 100644
--- a/.clang-ignore
+++ b/.clang-ignore
@@ -11,3 +11,4 @@
 ./valijson-prefix
 ./valijson-src
 ./valijson
+./subprojects