Use <endian.h>; OpenBSD doesn't have <byteswap.h>.

Index: io-webp.c
--- io-webp.c.orig
+++ io-webp.c
@@ -12,9 +12,7 @@
 
 #include "io-webp.h"
 
-#if __BYTE_ORDER__ == __ORDER_BIG_ENDIAN__
-#include <byteswap.h>
-#endif
+#include <endian.h>
 
 #define  IMAGE_READ_BUFFER_SIZE 65535
 
@@ -278,9 +276,7 @@ gdk_pixbuf__webp_anim_load_increment (gpointer      co
                 /* The next 4 bytes give the size of the webp container less the 8 byte header. */
                 uint32_t anim_size = *(uint32_t *) (buf + 4); /* gives file size not counting the first 8 bytes. */
 
-                #if __BYTE_ORDER__ == __ORDER_BIG_ENDIAN__
-		anim_size = bswap_32(anim_size);
-                #endif
+		anim_size = le32toh(anim_size);
 
                 uint32_t file_size = anim_size + 8;
                 if (file_size < size) {
