vlc_fixups: fix ssize_t not being defined

When compiling and not having writev/readv, the following error happens:

make[2]: Entering directory '/home/janniaux/Projects/videolabs/vlc/build-tsan/compat'
  CC       strlcpy.lo
  CC       strnstr.lo
In file included from ../config.h:952,
                 from ../../compat/strnstr.c:22:
../../include/vlc_fixups.h:232:1: error: unknown type name ‘ssize_t’; did you mean ‘size_t’?
  232 | ssize_t readv(int, const struct iovec *, int);
      | ^~~~~~~
      | size_t
In file included from ../config.h:952,
                 from ../../compat/strlcpy.c:22:
../../include/vlc_fixups.h:232:1: error: unknown type name ‘ssize_t’; did you mean ‘size_t’?
  232 | ssize_t readv(int, const struct iovec *, int);
      | ^~~~~~~
      | size_t
../../include/vlc_fixups.h:237:1: error: unknown type name ‘ssize_t’; did you mean ‘size_t’?
  237 | ssize_t writev(int, const struct iovec *, int);
      | ^~~~~~~
      | size_t
../../include/vlc_fixups.h:237:1: error: unknown type name ‘ssize_t’; did you mean ‘size_t’?
  237 | ssize_t writev(int, const struct iovec *, int);
      | ^~~~~~~
      | size_t
make[2]: *** [Makefile:1531: strnstr.lo] Error 1