package/gnupg: fix build with gcc 10

This commit backports an upstream patch made for gnupg2 into gnupg, in
order to fix build failures with gcc 10 due to the use of
-fno-common. Due to the code differences between upstream gnupg2 and
the old gnupg 1.x, the backport is in fact more a rewrite than an
actual backport.

Fixes:

  http://autobuild.buildroot.net/results/496a18833505dc589f7ae58f2c7e5fe80fe9af79/



Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>