libc: minimal: Add 'break' to suppress fall-through error in atoi.c

Compiling using clang fails with following error:

error: unannotated fall-through between switch labels
[-Werror,-Wimplicit-fallthrough]

Actually, missing 'break' statement is not an error here because
the next label is 'default' which contains 'break' only. For the same
reason we can add it to silence the compiler.

Signed-off-by: Patryk Duda <pdk@semihalf.com>