games/egoboo: fix build on big-endian architectures

md2.c:514:42: warning: implicit declaration of function 'SDL_swap32' is invalid in C99 [-Wimplicit-function-declaration]

cmd->data[i].index = SDL_swap32( cmd->data[i].s );

ld: error: undefined symbol: SDL_swap32

referenced by md2.c

md2.o:(md2_load)

cc: error: linker command failed with exit code 1 (use -v to see invocation)