package/tor: bump version to 0.2.7.6, fix musl build

According to
https://gitweb.torproject.org/tor.git/plain/ReleaseNotes?id=tor-0.2.7.6


"Tor 0.2.7.5 is the first stable release in the Tor 0.2.7 series."
so this patch bumps to the stable 2.7 series.

This patch also fixes a musl build error not yet found by the
autobuilders:

  CC       src/common/workqueue.o
src/common/sandbox.c:51:25: fatal error: bits/signum.h: Datei oder Verzeichnis nicht gefunden
 #include <bits/signum.h>
                         ^

Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>