Build chkjson on Travis

Merging CleverRaven#30759 broke the build by introducing a compile failure in
chkjson.  This should be built on Travis to allow detection of such
issues.

Previously chkjson was only built on Jenkins, but that doesn't happen
when the Makefile changes, so this problem went undetected.

build-scripts/build.sh

@@ -18,6 +18,10 @@ if [ -n "$TEST_STAGE" ]
 then
     build-scripts/lint-json.sh
     make -j "$num_jobs" style-json
+
+    # Also build chkjson (even though we're not using it), to catch any
+    # compile errors there
+    make -j "$num_jobs" chkjson
 elif [ -n "$JUST_JSON" ]
 then
     echo "Early exit on just-json change"