elflint: fix undefined 'buffer_left' reference

elflint: fix undefined 'buffer_left' reference

Link failure is reproducible on gcc-11.1.0 target:

```
$ autoreconf -i -f
$ ./configure --enable-maintainer-mode --disable-debuginfod \
    --host=x86_64-pc-linux-gnu \
    CFLAGS=-march=znver3 \
    CXXFLAGS=-march=znver3 \
    LDFLAGS=" "
$ make

  CCLD     elflint
ld: elflint.o: in function `check_attributes':
elflint.c:(.text+0xdcff): undefined reference to `buffer_left'
ld: elflint.c:(.text+0xe557): undefined reference to `buffer_left'
```

It happens due to possible external linkage of `buffer_left()`.

The change forces local linkage to always use local definition
(either inline or out-of-line).

Reported-by: Toralf Förster
Bug: https://bugs.gentoo.org/794601
Signed-off-by: Sergei Trofimovich <slyfox@gentoo.org>
Fixes: e95d1fbb ("elflint: Pull left() in file scope")
Signed-off-by: Dmitry V. Levin <ldv@altlinux.org>