Add hasmntopt(3)

bionic has the Linux-specific mntent.h but is missing hasmntopt().

Change-Id: I0ab7b83626c969704add4e64b37a6fc715d4a723
Signed-off-by: Greg Hackmann <ghackmann@google.com>
diff --git a/libc/libc.arm.map b/libc/libc.arm.map
index 5f7ae2d..57060ae 100644
--- a/libc/libc.arm.map
+++ b/libc/libc.arm.map
@@ -1274,6 +1274,7 @@
 LIBC_O {
   global:
     getdomainname;
+    hasmntopt;
     pthread_getname_np;
     setdomainname;
 } LIBC_N;