Makefile: add a prerequisite to the coverage-report target

Makefile: add a prerequisite to the coverage-report target

Directly invoking make coverage-report as a target results in an error because
its prerequisites are missing,

This patch adds the compile-test prerequisite, which is run only once each time
the compile-report target is invoked. In practice, the developer may decide to
review the coverage-report results without necessarily rerunning for this
coverage-test, if it has already been run.

Signed-off-by: Elia Pinto <gitter.spiros@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>