build: Find yaml-cpp only if building tools

Change-Id: I4c0d5cd67ea41f3285ccc1e203942222c1cf15fd
diff --git a/CMakeLists.txt b/CMakeLists.txt
index c26432e..b7cd7d0 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -51,7 +51,11 @@
             set(MSGPACK_TARGET "msgpackc-cxx")
         endif()
     endif()
-    find_package(yaml-cpp REQUIRED)
+
+    if (BUILD_TOOLS)
+        find_package(yaml-cpp REQUIRED)
+    endif()
+
     include_directories(${YAML_CPP_INCLUDE_DIR})
     find_package(fmt)
     pkg_search_module (opendht REQUIRED IMPORTED_TARGET opendht)