devel/p5-B-C: fix build

Threaded build currently fails:
mv C.xsc C.c

blib/arch/auto/B/C/C.so ---

chmod 755 blib/arch/auto/B/C/C.so

subdirs ---

mv: C.xsc: No such file or directory

Approved by: mat (mentor)
Differential Revision: https://reviews.freebsd.org/D20624