FindImageMagick: Allow including multiple times

This prevents attempting to create a duplicate target by adding an if(NOT TARGET ...) check as in other modules. The condition was missed in !8098 (merged).

I tested the change by placing the following CMakeLists.txt in Modules/ and running cmake -B build.

cmake_minimum_required(VERSION 3.20)

project(CheckCodeChanges
  DESCRIPTION "Confirm that FindImageMagick won't try to duplicate targets."
  LANGUAGES NONE
  VERSION 0.0.1.0
)

set(CMAKE_MODULE_PATH "${CMAKE_CURRENT_LIST_DIR}")

find_package(ImageMagick COMPONENTS Magick++)
find_package(ImageMagick COMPONENTS Magick++)

Before fix (b83fabd0):

-- Found ImageMagick: /opt/homebrew/Cellar/imagemagick/7.1.1-33/lib/libMagick++-7.Q16HDRI.dylib (found version "7.1.1-33")
CMake Error at FindImageMagick.cmake:188 (add_library):
  add_library cannot create imported target "ImageMagick::Magick++" because
  another target with the same name already exists.
Call Stack (most recent call first):
  FindImageMagick.cmake:235 (FIND_IMAGEMAGICK_API)
  CMakeLists.txt:12 (find_package)


-- Configuring incomplete, errors occurred!

After fix (1f54fa48):

jvanderzee@V22MYH9793 Modules % cmake -B temp                                                                                                                                                                                                        
-- Found ImageMagick: /opt/homebrew/Cellar/imagemagick/7.1.1-33/lib/libMagick++-7.Q16HDRI.dylib (found version "7.1.1-33")                                                                                                                           
-- Configuring done (0.2s)                                                                                                                                                                                                                           
-- Generating done (0.0s)                                                                                                                                                                                                                            
-- Build files have been written to: /Users/jvanderzee/cmake/Modules/temp  

Fixes: #25970 (closed)
Topic-rename: FindImageMagick-repeat