Support libdwarf prior to 5

Summary:
Not all systems have a libdwarf that supports dwarf 5. We only need a single tag from it, so just define it when needed.
Fixes: https://github.com/facebook/folly/issues/1654

Reviewed By: luciang

Differential Revision: D31423305

fbshipit-source-id: 89fecccb5ac68830a33b1fad51210cbaa9754bf8

folly/experimental/symbolizer/Dwarf.cpp

@@ -26,6 +26,12 @@
 
 #include <dwarf.h>
 
+// We need a single dwarf5 tag, but may not be building against
+// a new enough libdwarf, so just define it ourselves.
+#ifndef DW_TAG_skeleton_unit
+#define DW_TAG_skeleton_unit 0x4a
+#endif
+
 namespace folly {
 namespace symbolizer {