doc: fs: Add ingroup tag to fs_interface.h

Encapsulate definitions in fs_interface.h with ingroup file_system_api
doygen tag.

Signed-off-by: Dominik Ermel <dominik.ermel@nordicsemi.no>

include/fs/fs_interface.h

@@ -31,11 +31,17 @@ extern "C" {
 #endif /* filesystem selection */
 #endif /* CONFIG_FILE_SYSTEM_MAX_FILE_NAME */
 
+
 /* Type for fs_open flags */
 typedef uint8_t fs_mode_t;
 
 struct fs_mount_t;
 
+/**
+ * @addtogroup file_system_api
+ * @{
+ */
+
 /**
  * @brief File object representing an open file
  *
@@ -59,6 +65,10 @@ struct fs_dir_t {
 	const struct fs_mount_t *mp;
 };
 
+/**
+ * @}
+ */
+
 #ifdef __cplusplus
 }
 #endif