Update for C++ compiler compatibility

Update flash_map_backend.h to be compatible with a C++ compiler

Signed-off-by: Andrés Sánchez Pascual <tito97_sp@hotmail.com>
Signed-off-by: GitHub <noreply@github.com>

boot/nuttx/include/flash_map_backend/flash_map_backend.h

@@ -26,6 +26,10 @@
 
 #include <inttypes.h>
 
+#ifdef __cplusplus
+extern "C" {
+#endif
+  
 /****************************************************************************
  * Public Types
  ****************************************************************************/
@@ -419,4 +423,8 @@ int flash_area_id_to_multi_image_slot(int image_index, int area_id);
 
 int flash_area_id_from_image_offset(uint32_t offset);
 
+#ifdef __cplusplus
+}
+#endif
+
 #endif /* __BOOT_NUTTX_INCLUDE_FLASH_MAP_BACKEND_FLASH_MAP_BACKEND_H */