Fix unused return value warning for chdir()

It is good practice to not use void returns these days ;)

SkyFire_5xx/src/tools/map_extractor/System.cpp: In function â€˜void CreateDir(const string&)â€™:
SkyFire_5xx/src/tools/map_extractor/System.cpp:145:25: warning: ignoring return value of â€˜int chdir(const char*)â€™, declared with attribute warn_unused_result [-Wunused-result]