Add extra libc stubs for mntent related functions.

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

Change-Id: I6ccaa044145405cd7226c1c54a25d435c96a2308
diff --git a/libc/bionic/stubs.cpp b/libc/bionic/stubs.cpp
index 6edaa19..707036a 100644
--- a/libc/bionic/stubs.cpp
+++ b/libc/bionic/stubs.cpp
@@ -447,6 +447,16 @@
   return NULL;
 }
 
+FILE* setmntent(const char*, const char*) {
+  UNIMPLEMENTED;
+  return NULL;
+}
+
+int endmntent(FILE*) {
+  UNIMPLEMENTED;
+  return 1; /* Allways returns 1 according to man */
+}
+
 char* ttyname(int /*fd*/) { // NOLINT: implementing bad function.
   UNIMPLEMENTED;
   return NULL;