Remove unnecessary SDLCALL from IMG_xpm.c declarations

This made VC fail building.

The XPM declarations:

#if 0
#include "SDL_image.h"
#else
extern SDLCALL int SDLCALL IMG_isXPM(SDL_RWops *src);
extern SDLCALL SDL_Surface *IMG_LoadXPM_RW(SDL_RWops *src);
extern SDLCALL SDL_Surface * SDLCALL IMG_ReadXPMFromArray(const char **xpm);
#define IMG_SetError    SDL_SetError
#define IMG_GetError    SDL_GetError
#endif

SDLCALL defines:

/* By default SDL uses the C calling convention */
#ifndef SDLCALL
#if (defined(__WIN32__) || defined(__WINRT__)) && !defined(__GNUC__)
#define SDLCALL __cdecl
#elif defined(__OS2__) || defined(__EMX__)
#define SDLCALL _System
# if defined (__GNUC__) && !defined(_System)
#  define _System /* for old EMX/GCC compat.  */
# endif
#else
#define SDLCALL
#endif
#endif /* SDLCALL */

SDLCALL expands to nothing on GCC, but expands to __cdecl in VC, which threw that compiler off.

It's not necessary because we're not building these functions into a DLL. The previous version of the file did not use SDLCALL.

static inline SDL_Surface *IMG_ReadXPMFromArray(const char **xpm)
{
        return load_xpm(xpm, NULL);
}

With this fix, VC builds correctly.