Add flies (doomed #112) from the Hexen retail beta

...because it brought back memories of adding Strife support.
- The search function is radically different, but the flying mechanics are
  the same.

src/g_hexen/a_hexenmisc.cpp

@@ -38,6 +38,7 @@
 #include "a_fighterquietus.cpp"
 #include "a_firedemon.cpp"
 #include "a_flechette.cpp"
+#include "a_flies.cpp"
 #include "a_fog.cpp"
 #include "a_healingradius.cpp"
 #include "a_heresiarch.cpp"