Add extra libc stubs for mntent related functions.

This adds stubs for setmntent() and endmntent().

Change-Id: I6ccaa044145405cd7226c1c54a25d435c96a2308
diff --git a/libc/include/mntent.h b/libc/include/mntent.h
index 8b87f71..6cc0b18 100644
--- a/libc/include/mntent.h
+++ b/libc/include/mntent.h
@@ -30,7 +30,9 @@
 
 #include <stdio.h>
 #include <sys/cdefs.h>
+#include <paths.h>  /* for _PATH_MOUNTED */
 
+#define MOUNTED _PATH_MOUNTED
 #define MNTTYPE_IGNORE "ignore"
 
 struct mntent
@@ -48,6 +50,8 @@
 
 
 struct mntent* getmntent(FILE*);
+FILE* setmntent(const char*, const char*);
+int endmntent(FILE*);
 
 __END_DECLS