Documenation: call out lcov as required

Code coverage requires `lcov`, so update the docs to call it out
specifically.

Signed-off-by: Paul Fagerburg <pfagerburg@google.com>
Change-Id: Ie2898faa5188a7174c4e56ba34f1a4f02f939b03
Reviewed-on: https://review.coreboot.org/c/coreboot/+/58633
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Martin Roth <martinroth@google.com>

Documentation/technotes/2021-05-code-coverage.md

@@ -4,6 +4,9 @@ Code coverage for the coreboot unit tests allows us to see what lines of
 code in the coreboot library are covered by unit tests, and allows a test
 author to see where they need to add test cases for additional coverage.
 
+Code coverage requires `lcov`; install the tool if necessary by
+`sudo apt install lcov` or the equivalent for your system.
+
 Enable code coverage in your unit test build by setting the environment
 variable `COV` to 1; either `export COV=1` in your shell, or add it to your
 `make` command, e.g. `COV=1 make unit-tests`.