bindings/ruby: Fix ruby SWIG bindings generation

bindings/ruby: Fix ruby SWIG bindings generation

bindings/swig/doc.py generates a swig interface file for ruby bindings
that includes docstrings with comments braces ( /* and */ ) like this:

  %feature("docstring") sigrok::Channel::type "/* Type of this channel. */\n";
  %feature("docstring") sigrok::Channel::enabled "/* Enabled status of this channel. */\n";

SWIG generates *.cxx and adds its own braces to the docstring:

/*/* Document-class: Sigrok::Error
Exception thrown when an error code is returned by any libsigrok call. */
*/

this causes compilation error for Ruby bindings.

To fix the error we should not add extra braces to the docstring.
With this patch libsigrok compiles fine with with ruby 2.7 and swig 4.0.2.

Fixes bug #1526

Signed-off-by: Anatol Pomozov <redacted>