How to have inline functions in multiple files

My project has the following 4 files: main.c, rcm.h, rcm.c and queue.c.

In rcm.h I'm declaring all functions implemented in rcm.c and queue.c.

rcm.c looks like:

#include "rcm.h"

void rcm (void) {

  Queue *Q = init(10);

  /* Some other stuff */
}

queue.c` looks like:

#include "rcm.h"

extern inline Queue* init(int n) {
  return malloc(sizeof(Queue*);
}

and rcm.h:

#ifndef __RCM__
#define __RCM__

typedef struct queue { /*...*/ } Queue;

void rcm( void );

inline Queue* init( int n );
#endif

When compiling I get the the warnings

 gcc-7 -O0 -c rcm.c -o rcm.o
 In file included from rcm.c:15:0:
 rcm.h:58:15: warning: inline function 'init' declared but never defined
 inline Queue* init(int size);
               ^~~~
 gcc-7    -c queue.c -o queue.o
 gcc-7 main.c lib/rcm.o queue.o -o main
In file included from main.c:4:0:
inc/rcm.h:58:15: warning: inline function 'init' declared but never defined
 inline Queue* init(int size);
           ^~~~

But, when I am not declaring init() as inline compiles normally.