benchtests: Do not build bench-timing-type with MODULE_NAME=libc

Since commit 2682695e, `make bench-build' with `--enable-static-pie'
fails due to bench-timing-type being incorrectly built with MODULE_NAME
set to `libc'.  This commit sets MODULE_NAME to nonlib, thus fixing the
build failure.

Reviewed-by: Carlos O'Donell <carlos@redhat.com>