cmake: fix install cxx api headers

Close #1882

src/CMakeLists.txt

@@ -57,7 +57,7 @@ update_cached_list(MK_LINK_LIBRARIES ZLMediaKit::MediaKit)
 # 未在使用
 if(ENABLE_CXX_API)
   # 保留目录结构
-  install(DIRECTORY ${MediaKit_Root}/
+  install(DIRECTORY ${CMAKE_CURRENT_SOURCE_DIR}/
     DESTINATION ${INSTALL_PATH_INCLUDE}/ZLMediaKit
     REGEX ".*[.](md|cpp)$" EXCLUDE)
   install(TARGETS zlmediakit