fix missing header

[jemc]

This change was necessary in order to build with clang (on Ubuntu Linux x86).

[jemc]

For reference, these were the compilation errors that I saw, which were fixed by these changes:

/home/jemc/Documents/codex/MEC/mec-kontrol/api/KontrolModel.cpp: In member function ‘std::shared_ptr<Kontrol::Rack> Kontrol::KontrolModel::getRack(const Kontrol::EntityId&) const’:
/home/jemc/Documents/codex/MEC/mec-kontrol/api/KontrolModel.cpp:87:38: error: expected unqualified-id before ‘&’ token
   87 |     } catch (const std::out_of_range &) {
      |                                      ^
/home/jemc/Documents/codex/MEC/mec-kontrol/api/KontrolModel.cpp:87:37: error: expected ‘)’ before ‘&’ token
   87 |     } catch (const std::out_of_range &) {
      |             ~                       ^~
      |                                     )
/home/jemc/Documents/codex/MEC/mec-kontrol/api/KontrolModel.cpp:87:38: error: expected ‘{’ before ‘&’ token
   87 |     } catch (const std::out_of_range &) {
      |                                      ^
/home/jemc/Documents/codex/MEC/mec-kontrol/api/KontrolModel.cpp:87:39: error: expected primary-expression before ‘)’ token
   87 |     } catch (const std::out_of_range &) {
      |                                       ^
make[2]: *** [mec-kontrol/api/CMakeFiles/mec-kontrol-api.dir/build.make:146: mec-kontrol/api/CMakeFiles/mec-kontrol-api.dir/KontrolModel.cpp.o] Error 1
make[1]: *** [CMakeFiles/Makefile2:833: mec-kontrol/api/CMakeFiles/mec-kontrol-api.dir/all] Error 2
make: *** [Makefile:91: all] Error 2

[jemc]

For reference, this is my clang --version:

> clang --version
Ubuntu clang version 18.1.3 (1ubuntu1)
Target: x86_64-pc-linux-gnu
Thread model: posix
InstalledDir: /usr/bin