sdbus++-gen-meson: fix meson dependencies

When multiple YAML files are used (for example both interface and error)
only the `interface.yaml` was ending up in the generated `inputs` field
in the `custom_target`.  This made it so that changes in the other
YAML files were not causing rebuilds.

Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
Change-Id: Ib550aade282ceca97f31a435d51f35e7de592224
diff --git a/tools/sdbus++-gen-meson b/tools/sdbus++-gen-meson
index 2de2032..665c50e 100755
--- a/tools/sdbus++-gen-meson
+++ b/tools/sdbus++-gen-meson
@@ -230,7 +230,7 @@
     # Determine the source files based on the YAMLs present.
     sources=""
     for s in ${interfaces[$1]}; do
-        sources="'${yamldir}/$1.${s}', "
+        sources="${sources}'${yamldir}/$1.${s}', "
     done
 
     # Create the target to generate the interface.md file.