feat: fix nlohmann_json linkage from PRIVATE to PUBLIC (#270)

casbin/CMakeLists.txt

diff --git a/casbin/CMakeLists.txt b/casbin/CMakeLists.txt
index cc51e16..4ba5d11 100644
--- a/casbin/CMakeLists.txt
+++ b/casbin/CMakeLists.txt
@@ -77,7 +77,7 @@
 
 target_precompile_headers(casbin PRIVATE ${CASBIN_INCLUDE_DIR}/casbin/pch.h)
 target_include_directories(casbin PRIVATE ${CASBIN_INCLUDE_DIR})
-target_link_libraries(casbin PRIVATE nlohmann_json::nlohmann_json)
+target_link_libraries(casbin PUBLIC nlohmann_json::nlohmann_json)
 
 set_target_properties(casbin PROPERTIES 
     PREFIX ""