Mac OS X 10.5 compatibility (minor)

The build script for Mac OS X uses mdfind to locate the qt_menu.nib file.
In Mac OS X 10.5 Leopard, mdfind does not have the -name option.
This fix uses find instead of mdfind to locate qt_menu.nib.
With this fix, it becomes easier to build Electrum on Leopard, even if it is an old platform.