libidn: fix build without makeinfo

Build fails when the makeinfo utility is not installed on the host.

Fixes:
http://autobuild.buildroot.net/results/dfd/dfdfb34ed81ba3a4b7a7271be482e75eca849dbf/
http://autobuild.buildroot.net/results/b33/b33c0b0e6b1033ab1d1294a91b869ee6adcd391a/
http://autobuild.buildroot.net/results/940/9401cc10f6da6a2e3453ebc65ce573c370733fb5/



Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Tested-by: Bernd Kuhls <bernd.kuhls@t-online.de>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>