build: avoid build failures on gcc <= 10, or clang

build: avoid build failures on gcc <= 10, or clang

On gcc 10 the following build failure occurs:
  "error: a label can only be part of a statement
   and a declaration is not a statement"
This is because the current code is non standards conforming,
but GCC >= 11 will compile it (even with the -Wpedantic option).
This issue is tracked for GCC at:
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=111526

* src/tail.c (parse_options): Avoid a declaration after label,
by using a surrounding block.