moduleA.h

#ifndef MODULEA_H
#define MODULEA_H

#include "moduleB.h"

typedef struct
{
  ...
} ModuleAType;

void SomeFunctionA(ModuleAType* a, ModuleBType* b);

#endif /* MODULEA_H */

moduleB.h

#ifndef MODULEB_H
#define MODULEB_H

#include "moduleA.h"

typedef struct
{
  ModuleAType someField;  //   <----- this is the tricky bit
} ModuleBType;

void SomeFunctionB(ModuleAType* a, ModuleBType* b);

#endif /* MODULEB_H */

moduleB_fixed.h

#ifndef MODULEB_H
#define MODULEB_H

#include "moduleA.h"

struct ModuleBType_s;
typedef struct ModuleBType_s ModuleBType;
void SomeFunctionB(ModuleAType* a, ModuleBType* b);

#endif /* MODULEB_H */

moduleB_types.h

#ifndef MODULEB_TYPES_H
#define MODULEB_TYPES_H

#include "moduleA.h"

struct ModuleBType_s
{
  ModuleAType someField;
};

#endif /* MODULEB_TYPES_H */

somefile.c

#include "moduleA.h"

...

somefile_fixed.c

#include "moduleA.h"
#include "moduleB_types.h"

...

somefile_preprocessed.c

typedef struct
{
  ModuleAType someField;  //   <----- this is the tricky bit
} ModuleBType;

void SomeFunctionB(ModuleAType* a, ModuleBType* b);

typedef struct
{
  ...
} ModuleAType;

void SomeFunctionA(ModuleAType* a, ModuleBType* b);

...