libtest: Adding stopwatch.h into libtest_HEADERS list

[muzamel79]

We need this to be able to include the header externally.

[kira-syslogng]

This user does not have permission to start the build. Can one of the admins verify this patch and start the build?
(admin: you have the next options (make sure you checked the code):
"ok to test" to accept this pull request (and further changes) for testing
"test this please" for a one time test run
do nothing -> CI won't start)

[kira-syslogng]

This user does not have permission to start the build. Can one of the admins verify this patch and start the build?
(admin: you have the next options (make sure you checked the code):
"ok to test" to accept this pull request (and further changes) for testing
"test this please" for a one time test run
do nothing -> CI won't start)

[Kokan]

@kira-syslogng ok to test

[kira-syslogng]

Build SUCCESS