libxenstore: Use PTHREAD_STACK_MIN

libxenstore: Use PTHREAD_STACK_MIN

The existing value of 16K is smaller than the arm64 minimum stack size, which
is 128K. PTHREAD_STACK_MIN appears to be standard
http://pubs.opengroup.org/onlinepubs/009695399/functions/pthread_attr_setstacksize.html

Consindered setting a lower bound but the stack requirements of the watcher
thread are pretty minimal (tens of bytes from the looks of it) and unlikely to
blow PTHREAD_STACK_MIN on any useful platform.

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>