vp8/common: add extern "C" to headers

Change-Id: I13b434b1e6621e31962b08831c3587c039368c83

vp8/common/treecoder.h

@@ -12,6 +12,10 @@
 #ifndef VP8_COMMON_TREECODER_H_
 #define VP8_COMMON_TREECODER_H_
 
+#ifdef __cplusplus
+extern "C" {
+#endif
+
 typedef unsigned char vp8bc_index_t; /* probability index */
 
 
@@ -87,4 +91,8 @@ void vp8bc_tree_probs_from_distribution(
 );
 
 
+#ifdef __cplusplus
+}  // extern "C"
+#endif
+
 #endif  // VP8_COMMON_TREECODER_H_