Add further excludes to headerscheck

Some header files under contrib/isn/ are not meant to be included
independently, and they fail -Wmissing-variable-declarations when
doing so.

Reported-by: Thomas Munro <thomas.munro@gmail.com>
Discussion: https://www.postgresql.org/message-id/flat/CA%2BhUKG%2BYVt5MBD-w0HyHpsGb4U8RNge3DvAbDmOFy_epGhZ2Mg%40mail.gmail.com#aba3226c6dd493923bd6ce95d25a2d77

Author: petere

src/tools/pginclude/headerscheck

@@ -138,6 +138,12 @@ do
 	test "$f" = src/pl/tcl/pltclerrcodes.h && continue
 
 	# Also not meant to be included standalone.
+	test "$f" = contrib/isn/EAN13.h && continue
+	test "$f" = contrib/isn/ISBN.h && continue
+	test "$f" = contrib/isn/ISMN.h && continue
+	test "$f" = contrib/isn/ISSN.h && continue
+	test "$f" = contrib/isn/UPC.h && continue
+
 	test "$f" = src/include/common/unicode_nonspacing_table.h && continue
 	test "$f" = src/include/common/unicode_east_asian_fw_table.h && continue