fix: Return default value from ‘GetCacheTypeString’

The build fails with following message when -Werror
and -Werror=return-type are enabled.

In function ‘GetCacheTypeString’:
	error: control reaches end of non-void function [-Werror=return-type]

Simple fix is to return NULL from that function. The
return value is then checked by an assert in the
AddMapEntry. The assert will only work in debug mode,
but that seems to be enough.

src/utils/list_cpu_features.c

@@ -340,6 +340,8 @@ static Node* GetCacheTypeString(CacheType cache_type) {
     case CPU_FEATURE_CACHE_PREFETCH:
       return CreateConstantString("prefetch");
   }
+  /* Should never happen */
+  return NULL;
 }
 
 static void AddCacheInfo(Node* root, const CacheInfo* cache_info) {