Cannot place kernel's prototype in header

io.cuh:

#ifndef IO_CUH
#define IO_CUH

#include <cuda.h>

typedef struct{
    unsigned width;
    unsigned height;
    unsigned char *image; //RGBA
}rgb_image;

__global__ void transformToGrayKernel(rgb_image *img);
void decodeTwoSteps(const char* filename, rgb_image *img);
void encodeOneStep(const char* filename, rgb_image *img);
void processImage(const char *filename, rgb_image *img);

#endif // IO_CUH

I'm trying to compile simple program using the following makefile:

lodepng.o: lodepng.h
        cc -c lodepng.c -o lodepng.o
io.o: io.cuh lodepng.o
        nvcc -c io.cu -o io.o
main: io.o
        nvcc main.c io.o -o main

'main.c' uses one function from io.cu, which is dependent on lodepng.c.

After some minor warnings that reference to the code, I got the following error:

nvcc main.c io.o -o main nvcc warning : The 'compute_10' and 'sm_10' architectures are deprecated, and may be removed in a future release. In file included from main.c:1:0: io.cuh:12:12: error: expected ‘=’, ‘,’, ‘;’, ‘asm’ or ‘attribute’ before ‘void’ global void transformToGrayKernel(rgb_image *img); ^ makefile:6: recipe for target 'main' failed make: *** [main] Error 1