plotutils: fix parallel building

E.g. https://hydra.nixos.org/build/75429322:

     b_closepl.c:21:10: fatal error: xmi.h: No such file or directory
     make[2]: *** [Makefile:958: b_closepl.lo] Error 1
     make[2]: Leaving directory '/build/plotutils-2.6/libplot'