openbgpd: add -fcommon workaround
Workaround build failure on -fno-common toolchains like upstream
gcc-10. Otherwise build fails as:
ld: bgpd-rde_peer.o:/build/source/src/bgpd/bgpd.h:133: multiple definition of `bgpd_process';
bgpd-bgpd.o:/build/source/src/bgpd/bgpd.h:133: first defined here