Add getmntent_r to our collection of <mntent.h> compatibility stubs.

This helps build 'external/flo' out of the box.

Bug: 14841211
Change-Id: I30dde77239cceaf1f5743163744eb3604d27a266
diff --git a/libc/Android.mk b/libc/Android.mk
index d6c0599..5befacf 100644
--- a/libc/Android.mk
+++ b/libc/Android.mk
@@ -140,6 +140,7 @@
     bionic/mkdir.cpp \
     bionic/mkfifo.cpp \
     bionic/mknod.cpp \
+    bionic/mntent.cpp \
     bionic/open.cpp \
     bionic/pause.cpp \
     bionic/pipe.cpp \