🩹 guard install instruction (#531)

## Description

This PR fixes a small oversight in #529 where one of the install
instructions was not guarded properly. This should fix the issues
observed in cda-tum/mqt-qcec#352.

## Checklist:

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- [x] The pull request only contains commits that are related to it.
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project's style guidelines.

Signed-off-by: burgholzer <burgholzer@me.com>

src/zx/CMakeLists.txt

@@ -21,9 +21,11 @@ if(NOT TARGET ${MQT_CORE_TARGET_NAME}-zx)
                                         $<INSTALL_INTERFACE:${MQT_CORE_INCLUDE_INSTALL_DIR}>)
       add_library(MQT::Multiprecision ALIAS multiprecision)
 
-      # finally, we create install instructions for the respective header files
-      install(DIRECTORY ${MULTIPRECISION_INCLUDE_DIR}/boost
-              DESTINATION ${MQT_CORE_INCLUDE_INSTALL_DIR})
+      if(MQT_CORE_INSTALL)
+        # finally, we create install instructions for the respective header files
+        install(DIRECTORY ${MULTIPRECISION_INCLUDE_DIR}/boost
+                DESTINATION ${MQT_CORE_INCLUDE_INSTALL_DIR})
+      endif()
     endif()
   endif()