Fix ODR violation in ThemisPP (#576)

All functions defined in header files should 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 and I never learn because C++ is
such a monster that I easily forget something about it all the time.

src/wrappers/themis/themispp/secure_keygen.hpp

@@ -140,7 +140,7 @@ inline bool is_public_key(const std::vector<uint8_t>& key)
  * If the vector does not have enough space for a good key then
  * it will be resized. Otherwise no reallocations are performed.
  */
-void gen_sym_key(std::vector<uint8_t>& key)
+inline void gen_sym_key(std::vector<uint8_t>& key)
 {
     if (key.empty()) {
         size_t key_length = 0;
@@ -165,7 +165,7 @@ void gen_sym_key(std::vector<uint8_t>& key)
  *
  * @returns a newly allocated key of default size.
  */
-std::vector<uint8_t> gen_sym_key()
+inline std::vector<uint8_t> gen_sym_key()
 {
     std::vector<uint8_t> key;
     gen_sym_key(key);