https://bugs.gentoo.org/560212

pull in headers for funcs that are used

--- a/libdm/dm_handle2path.c
+++ b/libdm/dm_handle2path.c
@@ -20,6 +20,9 @@
 #include <dmapi_kern.h>
 #include "dmapi_lib.h"
 
+#include <fcntl.h> /* open */
+#include <unistd.h> /* close */
+#include <string.h> /* strlen */
 #include <mntent.h>
 #include <dirent.h>
 #ifdef linux
