quirc.h : Fix C++ function name mangling with extern C.

lib/quirc.h

@@ -19,6 +19,10 @@
 
 #include <stdint.h>
 
+#ifdef __cplusplus
+extern "C" {
+#endif
+
 struct quirc;
 
 /* Obtain the library version string. */
@@ -162,4 +166,8 @@ void quirc_extract(const struct quirc *q, int index,
 quirc_decode_error_t quirc_decode(const struct quirc_code *code,
 				  struct quirc_data *data);
 
+#ifdef __cplusplus
+}
+#endif
+
 #endif