Fix incompatibility with Make 4.3.
Fix incompatibility with Make 4.3.

+= does not prepend a space if the variable was empty.
Source: https://lwn.net/Articles/810071/

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