BSDfan666 is correct. MPLS changes to
ifconfig(8) were checked in for the first time in early in May, so it appears you were attempting to build
-current on a non
-current system. You should study Section 5.3 to find the appropriate upgrade path for whatever version you have installed:
http://openbsd.org/faq/faq5.html#BldBinary
If you do have 4.3
-release installed, you can upgrade to 4.3
-stable by using the OPENBSD_4_3 tag found in CVS. This is further described in Section 5.3.3:
http://openbsd.org/faq/faq5.html#BldGetSrc
Note if you are wanting to play with MPLS, you will only be able to do so in
-current.