Snippets for msgpack-c 6.1.0 from Alpine riscv64

Sourced from https://build.alpinelinux.org/buildlogs/build-edge-riscv64/community/msgpack-c/msgpack-c-6.1.0-r1.log.

• /home/buildozer/aports/community/msgpack-c/src/msgpack-c-6.1.0/example/boundary.c:51:16: warning: unused variable 'len' [-Wunused-variable]
     51 |         size_t len = strlen(#b);                             \
        |                ^~~
  

• /home/buildozer/aports/community/msgpack-c/src/msgpack-c-6.1.0/example/boundary.c:185:16: warning: unused variable 'len' [-Wunused-variable]
    185 |         size_t len = strlen(#b);                             \
        |                ^~~
  

• /home/buildozer/aports/community/msgpack-c/src/msgpack-c-6.1.0/example/boundary.c:39:12: warning: variable 'offset' set but not used [-Wunused-but-set-variable]
     39 |     size_t offset = 0;
        |            ^~~~~~