[big-endian] unbreak graphics/webp-pixbuf-loader

graphics/webp-pixbuf-loader failed in the ongoing powerpc bulk, but
this looks easy to fix. OpenBSD doesn't have <byteswap.h>, so use
<endian.h> (on big-endian platforms like powerpc and sparc64). With
this patch, "make package" works and "make test" passes on powerpc.

ok to commit?

../webp-pixbuf-loader-0.0.6/io-webp.c:16:10: fatal error: 'byteswap.h'
file not found
#include <byteswap.h>
^~~~~~~~~~~~
1 error generated.

Index: Makefile
===================================================================
RCS file: /cvs/ports/graphics/webp-pixbuf-loader/Makefile,v
retrieving revision 1.1.1.1
diff -u -p -r1.1.1.1 Makefile
--- Makefile 12 Aug 2022 11:42:55 -0000 1.1.1.1
+++ Makefile 22 Sep 2022 01:57:35 -0000
@@ -3,6 +3,7 @@ COMMENT= WebP GDK Pixbuf Loader library
GH_ACCOUNT= aruiz
GH_PROJECT= webp-pixbuf-loader
GH_TAGNAME= 0.0.6
+REVISION= 0

CATEGORIES= graphics

Index: patches/patch-io-webp_c
===================================================================
RCS file: patches/patch-io-webp_c
diff -N patches/patch-io-webp_c
--- /dev/null 1 Jan 1970 00:00:00 -0000
+++ patches/patch-io-webp_c 22 Sep 2022 01:57:35 -0000
@@ -0,0 +1,23 @@
+Use <endian.h> on BE_ARCHS; OpenBSD doesn't have <byteswap.h>.
+
+Index: io-webp.c
+--- io-webp.c.orig
++++ io-webp.c
+@@ -13,7 +13,7 @@
+ #include "io-webp.h"
+
+ #if __BYTE_ORDER__ == __ORDER_BIG_ENDIAN__
+-#include <byteswap.h>
++#include <endian.h>
+