iOS: Make sure QStandardPaths::displayName() is defined

The file qstandardpaths_ios.mm doesn't have an implementation for
this function, only (the wrongly named) qstandardpaths_mac.cpp
does. There's no Foundation API to get the directory name, so
we fall back to the hard-coded strings like all other platforms.

Change-Id: I6dcfeb6a0e5860dd0d4e9a0cd334b2c2181a0004
Reviewed-by: Richard Moe Gustavsen <richard.gustavsen@theqtcompany.com>