security/ipv6toolkit: fix build on riscv64

ld: error: can't create dynamic relocation R_RISCV_HI20 against symbol: .LCPI64_0 in readonly segment; recompile object files with -fPIC or pass '-Wl,-z,notext' to allow text relocations in the output

defined in /wrkdirs/usr/ports/security/ipv6toolkit/work/ipv6toolkit-v2.0/libipv6/libipv6.a(libipv6.o)
referenced by libipv6.c

libipv6.o:(time_diff_ms) in archive /wrkdirs/usr/ports/security/ipv6toolkit/work/ipv6toolkit-v2.0/libipv6/libipv6.a