cmake: check for nlohmann_json

Fixes: 070f60d system: get LLDP neighbors from JSON output of networkctl
Change-Id: I5e98def31a1546cc2f335306555a55771aeed3b3
diff --git a/CMakeLists.txt b/CMakeLists.txt
index bdd1ba1..19dcfa4 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -35,6 +35,7 @@
 find_package(sdbus-c++ "1.0.0" REQUIRED)
 find_package(PkgConfig)
 find_package(fmt)
+find_package(nlohmann_json REQUIRED)
 
 pkg_check_modules(SYSTEMD REQUIRED IMPORTED_TARGET libsystemd)
 if(${SYSTEMD_VERSION} VERSION_EQUAL 245)
@@ -233,6 +234,7 @@
         PkgConfig::LIBYANG
         PkgConfig::LIBNL
         SDBusCpp::sdbus-c++
+        nlohmann_json::nlohmann_json
     )
 
 # - firewall