gitignore: add meson subprojects and build dirs

When using meson build and no system-level sdbusplus you
end up with untracked files due to the sdbusplus subproject
extraction.  Add to the gitignore.

Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
Change-Id: Idf80042030ac40169641a38fc4c5dbbee51b2712
diff --git a/.gitignore b/.gitignore
index 4c0e7bb..799619f 100644
--- a/.gitignore
+++ b/.gitignore
@@ -1,4 +1,5 @@
-build/*
+build*/*
+subprojects/*/
 boost-src
 Boost-prefix
 CMakeFiles