net/l2tpd: Unbreak build with GCC 10 (-fno-common default)

/usr/local/bin/ld: pty.o:(.bss+0x0): multiple definition of `rand_source'; l2tpd.o:(.bss+0x530): first defined here
/usr/local/bin/ld: misc.o:(.bss+0x0): multiple definition of `rand_source'; l2tpd.o:(.bss+0x530): first defined here
/usr/local/bin/ld: control.o:(.bss+0x0): multiple definition of `rand_source'; l2tpd.o:(.bss+0x530): first defined here
/usr/local/bin/ld: avp.o:(.bss+0x0): multiple definition of `rand_source'; l2tpd.o:(.bss+0x530): first defined here
/usr/local/bin/ld: call.o:(.bss+0x0): multiple definition of `rand_source'; l2tpd.o:(.bss+0x530): first defined here
/usr/local/bin/ld: network.o:(.bss+0x140): multiple definition of `rand_source'; l2tpd.o:(.bss+0x530): first defined here
/usr/local/bin/ld: avpsend.o:(.bss+0x0): multiple definition of `rand_source'; l2tpd.o:(.bss+0x530): first defined here
/usr/local/bin/ld: scheduler.o:(.bss+0x8): multiple definition of `rand_source'; l2tpd.o:(.bss+0x530): first defined here
/usr/local/bin/ld: file.o:(.bss+0x228): multiple definition of `rand_source'; l2tpd.o:(.bss+0x530): first defined here
/usr/local/bin/ld: aaa.o:(.bss+0x0): multiple definition of `rand_source'; l2tpd.o:(.bss+0x530): first defined here
/usr/local/bin/ld: control.o: in function `handle_special':
control.c:(.text+0x220e): undefined reference to `check_control'
/usr/local/bin/ld: control.o: in function `handle_packet':
control.c:(.text+0x22d0): undefined reference to `check_payload'
/usr/local/bin/ld: control.c:(.text+0x22e6): undefined reference to `expand_payload'
/usr/local/bin/ld: control.c:(.text+0x231a): undefined reference to `check_control'
/usr/local/bin/ld: control.c:(.text+0x2414): undefined reference to `write_packet'

http://package18.nyi.freebsd.org/data/121amd64-default-PR244494/2020-06-10_18h04m44s/logs/errors/l2tpd-0.69_10.log

PR: 246700