qt: Fix musl build

qt: Fix musl build

This helps to make sure that QT_SOCKLEN_T is defined to be 'int' only for legacy
glibc < 2 and not also for other libraries which may define it as per standards
but are not glibc, e.g. musl.

Fixes the following build error:
In file included from ../../include/QtNetwork/private/qnet_unix_p.h:1:0,
                 from kernel/qnetworkinterface_unix.cpp:46:
../../include/QtNetwork/private/../../../src/network/socket/qnet_unix_p.h: In function 'int qt_safe_accept(int, sockaddr*, int*, int)':
../../include/QtNetwork/private/../../../src/network/socket/qnet_unix_p.h:121:76: error: invalid conversion from 'int*' to 'socklen_t* {aka unsigned int*}' [-fpermissive]

Fixes:
http://autobuild.buildroot.net/results/709/709d547699596d09b4601a7940677d2010adeedf/
http://autobuild.buildroot.net/results/f05/f0589e1d06404a7bd57f5734228afe45164e553d/

Backported from Qt5:
https://github.com/qtproject/qtbase/commit/813f468a14fb84af43c1f8fc0a1430277358eba2

Signed-off-by: Jörg Krause <joerg.krause@embedded.rocks>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>