make eigen dependency not conditional

Author: taminob

cmake/ExternalDependencies.cmake

@@ -22,16 +22,14 @@ if(BUILD_MQT_CORE_BINDINGS)
   find_package(nanobind CONFIG REQUIRED)
 endif()
 
-if(BUILD_MQT_CORE_MLIR)
-  FetchContent_Declare(
-    Eigen
-    GIT_REPOSITORY https://gitlab.com/libeigen/eigen.git
-    GIT_TAG 5.0.1
-    GIT_SHALLOW TRUE
-    FIND_PACKAGE_ARGS 3.4.0 # minimum version for locally installed package
-  )
-  list(APPEND FETCH_PACKAGES Eigen)
-endif()
+FetchContent_Declare(
+  Eigen
+  GIT_REPOSITORY https://gitlab.com/libeigen/eigen.git
+  GIT_TAG 5.0.1
+  GIT_SHALLOW TRUE
+  FIND_PACKAGE_ARGS 3.4.0 # minimum version for locally installed package
+)
+list(APPEND FETCH_PACKAGES Eigen)
 
 set(JSON_VERSION
     3.12.0