posix-aio-compat.c: fix warning with _FORTIFY_SOURCE

posix-aio-compat.c: fix warning with _FORTIFY_SOURCE

CC    posix-aio-compat.o
cc1: warnings being treated as errors
posix-aio-compat.c: In function 'aio_signal_handler':
posix-aio-compat.c:505: error: ignoring return value of 'write', declared with attribute warn_unused_result
make: *** [posix-aio-compat.o] Error 1

Signed-off-by: Kirill A. Shutemov <kirill@shutemov.name>
Signed-off-by: Juan Quintela <quintela@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>