core: make vector foreach standard C compliant

[MukundaKatta]

Summary

• avoid GNU void-pointer arithmetic in bf_vector_foreach
• use char-pointer arithmetic internally so the public macro compiles under standard C

Fixes #509.

Testing

• git diff --check
• cc -std=c17 -pedantic-errors -Werror -I src/libbpfilter/include -c /tmp/bpfilter-vector-XXXXXX.c

Note: full CMake configuration is blocked locally because libbpf is not installed.