Fix bug#24023: Let "ndb/src/common/portlib/gcc.cpp" export a dummy symbol.

ndb/src/common/portlib/gcc.cpp:
  Fix bug#24023:  Make sure this module always exports some symbol, whether used anywhere or not.

ndb/src/common/portlib/gcc.cpp

@@ -4,4 +4,7 @@
  */
 #ifdef DEFINE_CXA_PURE_VIRTUAL
 extern "C" { int __cxa_pure_virtual() { return 0;} }
+#else
+/* Some compiler/linker combinations fail on files without exported symbols. */
+extern "C" { int dummy_export_symbol() { return 0;} }
 #endif