Allow configuring cpack strip from command line (openvinotoolkit#6712)

Check if CPACK_STRIP_FILES configured before setting it
ON by default. This allows to configre it from command
line when required.

cmake/developer_package/packaging.cmake

@@ -53,7 +53,9 @@ macro(ie_cpack)
     set(CPACK_PACKAGE_VENDOR "Intel Corporation")
     set(CPACK_VERBATIM_VARIABLES ON)
     set(CPACK_COMPONENTS_ALL ${ARGN})
-    set(CPACK_STRIP_FILES ON)
+    if (NOT DEFINED CPACK_STRIP_FILES)
+        set(CPACK_STRIP_FILES ON)
+    endif()
     set(CPACK_THREADS 8)
 
     string(REPLACE "/" "_" CPACK_PACKAGE_VERSION "${CI_BUILD_NUMBER}")