[FIX] missed inlines in C++ binding

[topin89]

PR Description

Missed inline keyword before a couple of functions in bindings/cpp/iiopp.h . Since I included it in two .cpp files and no more "multiple definitions", that should be all missed inlines.

PR Type

• Bug fix (a change that fixes an issue)
• New feature (a change that adds new functionality)
• Breaking change (a change that affects other repos or cause CIs to fail)

PR Checklist

• I have conducted a self-review of my own code changes
• I have commented new code, particulary complex or unclear areas
• I have checked that I did not intoduced new warnings or errors (CI output)
• I have checked that components that use libiio did not get broken
• I have updated the documentation accordingly (GitHub Pages, READMEs, etc)

[dNechita]

Looks good, thank you for the fix!