graphics/xpaint: Fix build in current (llvm15)

PR:		270930
Reported by:	freebsd@oldach.net (maintainer)

graphics/xpaint/files/patch-rw_readWriteJP2K.c

+--- rw/readWriteJP2K.c.orig	2023-05-08 13:59:30 UTC
++++ rw/readWriteJP2K.c
+@@ -69,7 +69,7 @@ ReadJP2K(char *file_name)
+ 	int i, j, k, l, w, h, w0, nc, step, delta, shift, bpp;
+ 	int *src, codec = CODEC_JP2;
+ 
+-	if ((fp = fopen(file_name, "rb")) == NULL) return (-1);
++	if ((fp = fopen(file_name, "rb")) == NULL) return (Image *)(-1);
+ 
+ 	/* Read in the entire file */
+ 	fseek(fp, 0, SEEK_END);