Ensure util/check-make-install.in is exported

./configure needs util/check-make-install.in to be present in the source
directory in order to complete successfully.  Make sure this file is
included in source tarballs created from the repository.

.gitattributes

@@ -15,4 +15,5 @@ win32utils/**.txt eol=crlf
 /lib/lwres/man/resolver.5	 export-ignore
 /util/**			 export-ignore
 /util/bindkeys.pl		-export-ignore
+/util/check-make-install.in	-export-ignore
 /util/mksymtbl.pl		-export-ignore