Add -fcommon compiler flag for compatibility with GCC 10.

Compliant to policy 4.5.0

debian/control

@@ -10,7 +10,7 @@ Build-Depends:
  libg15-dev (>= 1.2.7-3),
  libg15render-dev (>= 1.2.0.svn250-2),
  libtool
-Standards-Version: 4.4.1
+Standards-Version: 4.5.0
 Homepage: https://sourceforge.net/projects/g15daemon/
 Vcs-Browser: https://salsa.debian.org/debian/g15daemon
 Vcs-Git: https://salsa.debian.org/debian/g15daemon.git