x11-toolkits/p5-Prima: fix build on 32-bits

img/codec_Xpm.c:635:21: error: incompatible function pointer types assigning to 'Bool (*)(PImgCodec, PImgLoadFileInstance)' (aka 'long (*)(struct ImgCodec *, struct _ImgLoadFileInstance *)') from 'int (PImgCodec, PImgLoadFileInstance)' (aka 'int (struct ImgCodec *, struct _ImgLoadFileInstance *)') [-Wincompatible-function-pointer-types]

vmt. load          = load;
                   ^ ~~~~

img/codec_Xpm.c:639:21: error: incompatible function pointer types assigning to 'Bool (*)(PImgCodec, PImgSaveFileInstance)' (aka 'long (*)(struct ImgCodec *, struct _ImgSaveFileInstance *)') from 'int (PImgCodec, PImgSaveFileInstance)' (aka 'int (struct ImgCodec *, struct _ImgSaveFileInstance *)') [-Wincompatible-function-pointer-types]

vmt. save          = save;
                   ^ ~~~~