Fix openvino build without OpenCV. (#5154)

* Fix openvino build without OpenCV.

* Don't call find_package in case of ENABLE_OPENCV=OFF

docs/snippets/CMakeLists.txt

@@ -15,7 +15,12 @@ if(NOT CLDNN__IOCL_ICD_INCDIRS OR TRUE)
 endif()
 
 # remove OpenCV related sources
-find_package(OpenCV QUIET)
+if (ENABLE_OPENCV)
+  find_package(OpenCV QUIET)
+else()
+  set(OpenCV_FOUND FALSE)
+endif()
+
 if(NOT OpenCV_FOUND)
     list(REMOVE_ITEM SOURCES "${CMAKE_CURRENT_SOURCE_DIR}/dldt_optimization_guide5.cpp"
                              "${CMAKE_CURRENT_SOURCE_DIR}/ShapeInference.cpp")