math/mprime: Fix build on i386 by adding USES=localbase

The gmp.h header wasn't found because /usr/local/include wasn't
in the search path on i386.

Reported by: fallout
Approved by: portmgr (blanket unbreak)

(cherry picked from commit 97a5d4ce8c6d159f1101aaa5871204f0afad5702)