Changes: Include strings.h where bzero came from

Fix for:

 CXX(target) Release/obj.target/pdfutils/src/document.o
../src/document.cc: In member function ‘v8::Handle<v8::Value> Document::getProperty(const char*)’:
../src/document.cc:248:31: error: ‘bzero’ was not declared in this scope
make: *** [Release/obj.target/pdfutils/src/document.o] Error 1