Added "extern C" blocks to textencode. (#244)

* Added "extern C" blocks to textencode.

* Added accidentally-deleted test_upb.lua, deleted unneeded test.proto.

tests/bindings/lua/test.proto

@@ -1,7 +0,0 @@
-syntax = "proto2";
-
-package lua_test;
-
-message TestLua {
-  map<string, double> map_string_double = 1;
-}

upb/textencode.h

@@ -4,6 +4,10 @@
 
 #include "upb/def.h"
 
+#ifdef __cplusplus
+extern "C" {
+#endif
+
 enum {
   /* When set, prints everything on a single line. */
   UPB_TXTENC_SINGLELINE = 1,
@@ -24,4 +28,8 @@ size_t upb_textencode(const upb_msg *msg, const upb_msgdef *m,
                       const upb_symtab *ext_pool, int options, char *buf,
                       size_t size);
 
+#ifdef __cplusplus
+}  /* extern "C" */
+#endif
+
 #endif  /* UPB_TEXTENCODE_H_ */