[winpr,utils] fix cJSON detection

pkg-config sets CJSON_FOUND, check for that instead of the paths, which
might be unset or set to <var>-NOTFOUND

winpr/libwinpr/utils/CMakeLists.txt

@@ -202,7 +202,7 @@ if (NOT WITH_JSON_DISABLED)
 			pkg_check_modules(CJSON libcjson)
 		endif()
 
-		if (NOT CJSON_LIBRARIES OR NOT CJSON_INCLUDE_DIRS)
+		if (NOT CJSON_FOUND)
 			find_path(CJSON_INCLUDE_DIRS
 				NAMES cjson/cJSON.h
 			)