Adding volatile qualifer

While looking into the Pgpool-II main source code, I noticed that
local pointer array "my_backend_status" is not volatile qualified,
while it should have been.

BACKEND_STATUS *my_backend_status[MAX_NUM_BACKENDS]; /* Backend status buffer */

This array members are initialized:

for (i = 0; i < MAX_NUM_BACKENDS; i++)
{
my_backend_status[i] = &(BACKEND_INFO(i).backend_status);
}

Without volatile, *(my_backend_status[i]) could read state value
because of compiler optimization. Since my_backend_status[i] is
referred to in popular VALID_BACKEND macro, we should fix it in all
supported branches.

Attached is the patch for master branch.

Regards,
--
Tatsuo Ishii
SRA OSS K.K.
English: http://www.sraoss.co.jp/index_en/
Japanese:http://www.sraoss.co.jp