xts5: startup: avoid multiple definition errors

Avoid errors like:

/usr/bin/ld: xts5/src/lib/libxtest.a.p/startup.c.o:/xts5/src/lib/startup.c:168: multiple definition of `ErrdefGC'; xts5/src/lib/libxtest.a.p/ex_startup.c.o:/xts5/src/lib/ex_startup.c:140: first defined here
/usr/bin/ld: xts5/src/lib/libxtest.a.p/startup.c.o:/xts5/src/lib/startup.c:169: multiple definition of `ErrdefColormap'; xts5/src/lib/libxtest.a.p/ex_startup.c.o:/xts5/src/lib/ex_startup.c:141: first defined here