cpu/esp32: fix C linkage compilatio error

With the new toolchain version required to fix issue RIOT-OS#13133, the compilation of `examples/posix_socket` fails due to a C linkage error in `atomic_base.h`. The reason is that including `drivers/include/mtd.h` in `boards/esp32/board_common.h` inside the `extern C` block finally leads to including `atomic_base.h` inside the `extern C` block which in turn to the C linkage error for the template definitions in this file.

boards/common/esp32/include/board_common.h

@@ -35,8 +35,12 @@
 #include "periph/gpio.h"
 #include "sdk_conf.h"
 
+#if MODULE_MTD
+#include "mtd.h"
+#endif
+
 #ifdef __cplusplus
- extern "C" {
+extern "C" {
 #endif
 
 /**
@@ -103,7 +107,6 @@
  * Built-in SPI flash memory is used as MTD system drive.
  * @{
  */
-#include "mtd.h"
 
 /**
  * @brief   MTD drive start address in SPI flash memory

cpu/esp32/include/sdk_conf.h

@@ -24,12 +24,12 @@
 
 #ifndef DOXYGEN
 
+#include "board.h"
+
 #ifdef __cplusplus
 extern "C" {
 #endif
 
-#include "board.h"
-
 /**
  * @brief   Defines the CPU frequency [values = 2, 40, 80, 160 and 240]
  */