Avoid implicit const cast in GDALPDFObjectPoppler::GetString()

[Algunenano]

Just pleasing the compiler (gcc 7.3.1)

[rouault]

This breaks with older version of Poppler

pdfobject.cpp:1060:93: error: passing ‘const GooString’ as ‘this’ argument of ‘int GooString::getLength()’ discards qualifiers [-fpermissive]
                                                       static_cast<int>(gooString->getLength())));

Probably this should be conditionalized by the appropriate version of Poppler where getLength() has been tagged as const

Or simpler, but less elegant, do

GooString* gooString = const_cast<GooString*>(m_po->getString());

[Algunenano]

Works for me too. Thanks a lot!