package/python-rpi-gpio: fix gcc-10 compatibility patch

Although the patch makes the package build OK, it fails at runtime
when the module is imported, because of missing symbols:

    ImportError: /usr/lib/python3.8/site-packages/RPi/_GPIO.cpython-38-aarch64-linux-gnu.so: undefined symbol: high

Fix that by making sure the symbols are declared once, but only once.

Fixes: #13166

Reported-by: Christian Stewart <christian@paral.in>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>
Cc: Michael Fischer <mf@go-sys.de>
Cc: Asaf Kahlon <asafka7@gmail.com>
Cc: Yegor Yefremov <yegorslists@googlemail.com>
Cc: Ian Haylock <haylocki@yahoo.co.uk>
Tested-by: Christian Stewart <christian@paral.in>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>