Add missing extern "C" to header files (closes #104)

src/rp2_common/hardware_divider/include/hardware/divider.h

@@ -36,6 +36,10 @@
  * \include hello_divider.c
  */
 
+#ifdef __cplusplus
+extern "C" {
+#endif
+
 typedef uint64_t divmod_result_t;
 
 /*! \brief Start a signed asynchronous divide
@@ -392,4 +396,8 @@ void hw_divider_save_state(hw_divider_state_t *dest);
 
 void hw_divider_restore_state(hw_divider_state_t *src);
 
+#ifdef __cplusplus
+}
+#endif
+
 #endif // _HARDWARE_DIVIDER_H