BUILD/MINOR: fix Lua build on Mac OS X (again)

Previous commit (13113d6 "MINOR/BUILD: fix Lua build on Mac OS X")
contains a typo, it uses "-export-dynamic" instead of "-export_dynamic"
(dash instead of underscore), despite what the commit message suggests,
and it obviously doesn't work. Thanks to Kirill A. Korinsky for reporting
it.

This patch should be backported on each version from 1.6 like the
aforementionned one above.
diff --git a/Makefile b/Makefile
index db3a7d1..217dcd3 100644
--- a/Makefile
+++ b/Makefile
@@ -331,7 +331,7 @@
   USE_POLL       = implicit
   USE_KQUEUE     = implicit
   USE_TPROXY     = implicit
-  EXPORT_SYMBOL  = -export-dynamic
+  EXPORT_SYMBOL  = -export_dynamic
 else
 ifeq ($(TARGET),openbsd)
   # This is for OpenBSD >= 5.7