[app][tests] fix a function definition to make the prototype
[app][tests] fix a function definition to make the prototype

Only shows up when trying to link with LTO.

travisg/lk,travisg/lk,travisg/lk,littlekernel/lk,littlekernel/lk,travisg/lk,littlekernel/lk,hollanderic/lkstuff,littlekernel/lk,hollanderic/lkstuff,hollanderic/lkstuff,hollanderic/lkstuff