Use C99 designated initialisers for PerlIO_funcs

[ilmari]

This avoids having to count NULLs for layers that only implement a subset of functions.

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• This set of changes does not require a perldelta entry.

[tonycoz]

I think it is a silly warning with designated initializers, but g++ produces a mess of warnings:

g++ -c -DPERL_CORE -fwrapv -DDEBUGGING -fno-strict-aliasing -pipe -fstack-protector-strong -I/usr/local/include -D_LARGEFILE_SOURCE -D_FILE_OFFSET_BITS=64 -O2 -g -Wall -Werror=pointer-arith -Werror=vla -Wextra -Wno-long-long -Wno-nonnull-compare -Wwrite-strings -Wno-use-after-free perlio.c
perlio.c:1031:1: warning: missing initializer for member ‘_PerlIO_funcs::size’ [-Wmissing-field-initializers]
 1031 | };
      | ^
perlio.c:1031:1: warning: missing initializer for member ‘_PerlIO_funcs::Popped’ [-Wmissing-field-initializers]
perlio.c:1031:1: warning: missing initializer for member ‘_PerlIO_funcs::Binmode’ [-Wmissing-field-initializers]
perlio.c:1031:1: warning: missing initializer for member ‘_PerlIO_funcs::Getarg’ [-Wmissing-field-initializers]
... and many more

Adding C++ default initializers prevents the warnings but that would be a mess of macros for us.

I suspect the best choice would be to suppress the warning:

GCC_DIAG_IGNORE_DECL(-Wmissing-field-initializers);
PERLIO_FUNCS_DECL(PerlIO_remove) = {
     .fsize = sizeof(PerlIO_funcs),
    .name = "pop",
    .kind = PERLIO_K_DUMMY | PERLIO_K_UTF8,
    .Pushed = PerlIOPop_pushed,
    .Open = PerlIOBase_open,
};
GCC_DIAG_RESTORE