package: libpng: don't append prefix to symbol names in the version script file

Even if Blackfin GNU toolchain add prefix '_' to all symbols,
symbol prefix is not accepted in the link flag --version-script.
Don't append prefix in the symbols in the version script file.

Signed-off-by: Sonic Zhang <sonic.zhang@analog.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>