Fix ODR violation in ThemisPP

[ilammy]

All functions defined in header files must be marked inline so that the compiler does proper deduplication and does not violate “one definition rule” (ODR). Otherwise (you guessed it, right?)
undefined behavior happens.

Thanks goes go clang-analyzer whose warnings we diligently ignore. We've been burnt by this in the past (see #540, and there were customers with broken builds because of this). I never learn because C++ is such a monster that I easily forget something about it all the time.

This is new symmetric key generation API that is pending for release in Themis 0.13.

Checklist

• Change is covered by automated tests
• The coding guidelines are followed
• Public API has proper documentation

[Lagovas]

where we define it in several places? or it conflicts with functions from C library?

[ilammy]

where we define it in several places?

We don‘t but the users will. If ThemisPP is used in multiple translation units (say, A.cpp and B.cpp) both of which #include <themispp/secure_keygen.hpp> then both of them will “define” this function — ThemisPP being header-only library provides a definition, not just declaration. If it’s not marked inline then this might lead to a linker error caused by multiple conflicting definitions of a symbol. inline can be used to tell the linker to ignore duplicates.

[vixentael]

good job! some of our users caught this error