build: add find_package(Threads REQUIRED) for replxx dep chain

replxx's cmake config propagates Threads::Threads; without an explicit
find_package(Threads) CMake errors out during generate step.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: Vaniell0

CMakeLists.txt

@@ -124,6 +124,7 @@ endif()
 option(IRIS_IRISH "Build the irish interpreter binary" OFF)
 
 if(IRIS_IRISH)
+    find_package(Threads REQUIRED)
     find_package(replxx QUIET HINTS
         "${replxx_ROOT}/share/cmake/replxx"
         "$ENV{replxx_ROOT}/share/cmake/replxx"