Restore STDERR in pngtest.c

[bungeman]

In "test: Add consistency checks for the PNG_LIBPNG_VER* number" [0] the STDERR macro was moved from outside an ifdef to inside an ifdef. This broke the code in the else of this ifdef which also uses the STDERR macro. Move STDERR back to where it was to avoid compile errors in the else case.

[0] https://github.com/pnggroup/libpng/commit/cc8006c48d90cca8bf380fa69469b08f4edb00c5

Fixes: https://github.com/pnggroup/libpng/issues/560