nginx: bump to version 1.15.7

nginx: bump to version 1.15.7

1.15.7 contains a number of bugfixes. From the changes file:

     *) Bugfix: memory leak on errors during reconfiguration.

    *) Bugfix: in the $upstream_response_time, $upstream_connect_time, and
       $upstream_header_time variables.

    *) Bugfix: a segmentation fault might occur in a worker process if the
       ngx_http_mp4_module was used on 32-bit platforms.

https://nginx.org/en/CHANGES

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>