package/qt5: needs host gcc >= 5.0 for full C++11

Building qmake requires full C++11, which boils down to gcc >= 5.0,
which is what upstream advertises as a requirement anyway:

    https://doc.qt.io/qt-5.15/supported-platforms.html

    Distribution   | Architecture     | Compiler
    Generic Linux  | x86 and x86_64   | GCC (5 or later), ICC 18.x

Fixes:
    http://autobuild.buildroot.org/results/c3e/c3ee971a72f268e72b69a647e8fd00a8cee7dc91/
    http://autobuild.buildroot.org/results/89c/89c9a88b4e1195e952528574263201d4fbc27570/


    [...]

Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>
Cc: Peter Seiderer <ps.report@gmx.net>
Cc: Julien Corjon <corjon.j@ecagroup.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>