cstyle: Allow URLs in C++ comments

If a C++ comment contained a URL, the :// part of the URL would
trigger an error because there was no trailing blank, but trailing
blanks make for an invalid URL. Modify the check to ignore text
within the C++ comment.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Chris Lindee <chris.lindee+github@gmail.com>
Closes #13987