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

src/rp2_common/hardware_flash/include/hardware/flash.h

@@ -44,6 +44,9 @@
  * \include flash_program.c
  */
 
+#ifdef __cplusplus
+extern "C" {
+#endif
 
 /*! \brief  Erase areas of flash
  *  \ingroup hardware_flash
@@ -75,4 +78,8 @@ void flash_range_program(uint32_t flash_offs, const uint8_t *data, size_t count)
  */
 void flash_get_unique_id(uint8_t *id_out);
 
+#ifdef __cplusplus
+}
+#endif
+
 #endif