uri: fix uri.c debug info for lcov

lcov reports the following warnings:

Cannot open source file src/uri.rl
Cannot open source file src/uri.c

coveralls-lcov then fails with this message:

coveralls-lcov --service-name travis-ci --service-job-id 498721113 --repo-token [FILTERED] coverage.info
/var/lib/gems/2.3.0/gems/coveralls-lcov-1.5.1/lib/coveralls/lcov/converter.rb:63:in `initialize': No such file or directory @ rb_sysopen - /tarantool/src/lib/uri/CMakeFiles/uri.dir/src/uri.c (Errno::ENOENT)
	from /var/lib/gems/2.3.0/gems/coveralls-lcov-1.5.1/lib/coveralls/lcov/converter.rb:63:in `open'
	from /var/lib/gems/2.3.0/gems/coveralls-lcov-1.5.1/lib/coveralls/lcov/converter.rb:63:in `generate_source_file'
	from /var/lib/gems/2.3.0/gems/coveralls-lcov-1.5.1/lib/coveralls/lcov/converter.rb:16:in `block in convert'
	from /var/lib/gems/2.3.0/gems/coveralls-lcov-1.5.1/lib/coveralls/lcov/converter.rb:15:in `each'
	from /var/lib/gems/2.3.0/gems/coveralls-lcov-1.5.1/lib/coveralls/lcov/converter.rb:15:in `convert'
	from /var/lib/gems/2.3.0/gems/coveralls-lcov-1.5.1/lib/coveralls/lcov/runner.rb:68:in `run'
	from /var/lib/gems/2.3.0/gems/coveralls-lcov-1.5.1/bin/coveralls-lcov:5:in `<top (required)>'
	from /usr/local/bin/coveralls-lcov:22:in `load'
	from /usr/local/bin/coveralls-lcov:22:in `<main>'

So coverage target in Travis-CI fails and a coverage does not reported
to coveralls.io.

The bug was introduced in d735b6b (move
'uri' lib to src/lib/).