musl-libc compatibility

[kit-ty-kate]

The latest version of core_unix (v0.17.0) fails on Alpine Linux with (amongst other error messages):

linux_ext_stubs.c: In function 'core_linux_sendmsg_nonblocking_no_sigpipe_stub':
linux_ext_stubs.c:239:45: error: initialization of 'int' from 'void *' makes integer from pointer without a cast [-Wint-conversion]
  239 |   struct msghdr msghdr = {NULL, 0, NULL, 0, NULL, 0, 0};
      |                                             ^~~~
linux_ext_stubs.c:239:45: note: (near initialization for 'msghdr.__pad1')

[copy]

Fix in #12