packages: add OS400/chkstrings.c to the dist

Reported-by: Jon Rumsey
Fixes #5142
Closes #5143

packages/Makefile.am

@@ -6,6 +6,7 @@ EXTRA_DIST = README \
   OS400/README.OS400 \
   OS400/ccsidcurl.c \
   OS400/ccsidcurl.h \
+  OS400/chkstrings.c \
   OS400/curl.inc.in \
   OS400/initscript.sh \
   OS400/make-include.sh \