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/tests/Android.mk b/tests/Android.mk
index 10e288c..b17492a 100644
--- a/tests/Android.mk
+++ b/tests/Android.mk
@@ -59,6 +59,7 @@
     locale_test.cpp \
     malloc_test.cpp \
     math_test.cpp \
+    mntent_test.cpp \
     netdb_test.cpp \
     pthread_test.cpp \
     regex_test.cpp \