Fix GCC's "only here as ‘friend’" warning about swap()

The compiler warning reads:
warning: ‘void KDevelop::swap(KDevelop::IndexedString&, KDevelop::IndexedString&)’ has not been declared within ‘KDevelop’
  306 | void KDevelop::swap(IndexedString& a, IndexedString& b) noexcept
note: only here as a ‘friend’
  170 |     friend KDEVPLATFORMSERIALIZATION_EXPORT void swap(IndexedString&, IndexedString&) noexcept;

A relevant Stack Overflow question:
https://stackoverflow.com/questions/44749878/warning-defining-friend-operator-declared-inside-a-namespace