clk.h: add extern C guards

Closes https://github.com/espressif/esp-idf/issues/4215

components/esp32s2beta/include/esp32s2beta/clk.h

@@ -13,6 +13,11 @@
 // limitations under the License.
 
 #pragma once
+#include <stdint.h>
+
+#ifdef __cplusplus
+extern "C" {
+#endif
 
 /**
  * @file esp_clk.h
@@ -73,3 +78,7 @@ int esp_clk_apb_freq(void);
  * @return Value or RTC counter, expressed in microseconds
  */
 uint64_t esp_clk_rtc_time(void);
+
+#ifdef __cplusplus
+}
+#endif