Fix tst-ftell-active-handler.c warning.

A recent change to libio/tst-ftell-active-handler.c (postdating my
last check for warnings on x86) introduced a format warning from a
long int variable used with a %zu format.  This patch fixes it by
using %ld for the format to match the variable.

Tested for x86.

	* libio/tst-ftell-active-handler.c (do_ftruncate_test): Use %ld
	format for long int variable.