camera: Provide correct forward-declaration of camera_imageformat_type

This resolves the build error
```
arm-none-eabi-ld: build-spresense/libmpy.a(Camera.o):(.rodata.camera_imageformat_type+0x0): multiple definition of `camera_imageformat_type'; build-spresense/libmpy.a(__init__.o):(.rodata.camera_imageformat_type+0x0): first defined here
arm-none-eabi-ld: build-spresense/libmpy.a(ImageFormat.o):(.rodata.camera_imageformat_type+0x0): multiple definition of `camera_imageformat_type'; build-spresense/libmpy.a(__init__.o):(.rodata.camera_imageformat_type+0x0): first defined here
arm-none-eabi-ld: build-spresense/libmpy.a(Camera.o):(.rodata.camera_imageformat_type+0x0): multiple definition of `camera_imageformat_type'; build-spresense/libmpy.a(__init__.o):(.rodata.camera_imageformat_type+0x0): first defined here
```

Author: jepler

shared-bindings/camera/ImageFormat.c

@@ -38,7 +38,6 @@
 //|     RGB565: ImageFormat
 //|     """RGB565 format."""
 //|
-const mp_obj_type_t camera_imageformat_type;
 
 const camera_imageformat_obj_t camera_imageformat_jpg_obj = {
     { &camera_imageformat_type },

shared-bindings/camera/ImageFormat.h

@@ -35,7 +35,7 @@ typedef enum {
     IMAGEFORMAT_RGB565,
 } camera_imageformat_t;
 
-const mp_obj_type_t camera_imageformat_type;
+extern const mp_obj_type_t camera_imageformat_type;
 
 camera_imageformat_t camera_imageformat_obj_to_type(mp_obj_t obj);
 mp_obj_t camera_imageformat_type_to_obj(camera_imageformat_t mode);