visibility of rmmard_() in CONEX set to hidden

This remedies a clash of symbols with EPOS, which has its own
implementation of rmmard_(). For some reason, EPOS would still
call the CONEX implementation, though.

modules/conex/conex_random.cpp

@@ -2,6 +2,7 @@
 
 IMPLEMENT_RNG(conex)
 
-extern "C" void rmmard_(double field[], int const* N, int*) {
+extern "C" __attribute__((visibility("hidden"))) void rmmard_(double field[],
+                                                              int const* N, int*) {
   for (int i = 0; i < *N; ++i) { field[i] = draw_std_rnd(); }
 }