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

src/rp2_common/hardware_claim/include/hardware/claim.h

@@ -32,6 +32,10 @@
  * 3. Finding unused resources
  */
 
+#ifdef __cplusplus
+extern "C" {
+#endif
+
 /*! \brief Atomically claim a resource, panicking if it is already in use
  *  \ingroup hardware_claim
  *
@@ -98,4 +102,8 @@ uint32_t hw_claim_lock(void);
  */
 void hw_claim_unlock(uint32_t token);
 
+#ifdef __cplusplus
+}
+#endif
+
 #endif