[link-metrics] make union packed to fix warning (#11920)

Clang warns that mMetricsValue is less aligned that its
union type. Make the union packed to fix this.

Signed-off-by: Damian Krolik <[email protected]>

src/core/thread/link_metrics_tlvs.hpp

@@ -153,7 +153,7 @@ public:
 
 private:
     uint8_t mMetricsTypeId;
-    union
+    union OT_TOOL_PACKED_FIELD
     {
         uint8_t  m8;
         uint32_t m32;