[gn] expand dummy target from 2ed813f

It also needs a sources list for the sync script not to complain.

Author: nico

llvm/utils/gn/secondary/llvm/unittests/Target/VE/BUILD.gn

@@ -1 +1,4 @@
 # FIXME: If we add the VE target to the GN build, add VETests here.
+group("dummy") {
+  sources = [ "MachineInstrTest.cpp" ]
+}