Adds import to fix cassert import error

[Versus7]

While building Iridescence for use on Linux, I came across the same error that 'assert' is not declared in this context.

/iridescence/src/glk/pointcloud_buffer.cpp:120:3: error: ‘assert’ was not declared in this
 scope                                                                                               
  120 |   assert(this->num_points == num_points);                                                    
      |   ^~~~~~                                                                                     
/iridescence/src/glk/pointcloud_buffer.cpp:10:1: note: ‘assert’ is defined in header ‘<cas
sert>’; did you forget to ‘#include <cassert>’?                                                      

This PR adds the missing #include directive to fix the compilation error.

Notes

• This is a minimal fix for a build issue on Linux.
• It closes error: use of undeclared identifier 'assert' #149
• This change mirrors the same fix already included in the in-progress PR for macOS support (Enable build and install on macos #153), but is proposed here as a standalone fix for earlier resolution.

[koide3]

Thanks!