properly prefix ${X11INCDIR} uses with ${DESTDIR}.

this fixes a build issue seen where the host pixman.h was included,
and has been problem ever since tsutsui fixed my original code (from
2008) about 29 months ago.  i'm surprised this has not been seen
anywhere else in that time.

XXX: pullup-10

share/mk/bsd.x11.mk

@@ -1,4 +1,4 @@
-#	$NetBSD: bsd.x11.mk,v 1.146 2022/12/19 01:51:29 mrg Exp $
+#	$NetBSD: bsd.x11.mk,v 1.147 2022/12/21 20:09:35 mrg Exp $
 
 .include <bsd.init.mk>
 
@@ -46,8 +46,8 @@ X11FLAGS.EXTENSION=	${X11FLAGS.BASE_EXTENSION} \
 X11FLAGS.DIX=		-DHAVE_DIX_CONFIG_H -D_BSD_SOURCE -DHAS_FCHOWN \
 			-DHAS_STICKY_DIR_BIT -D_POSIX_THREAD_SAFE_FUNCTIONS=200112L \
 			-DHAVE_XORG_CONFIG_H
-X11INCS.DIX=		-I${X11INCDIR}/freetype2  \
-			-I${X11INCDIR}/pixman-1 \
+X11INCS.DIX=		-I${DESTDIR}${X11INCDIR}/freetype2  \
+			-I${DESTDIR}${X11INCDIR}/pixman-1 \
 			-I$(X11SRCDIR.xorg-server)/include \
 			-I$(X11SRCDIR.xorg-server)/Xext \
 			-I$(X11SRCDIR.xorg-server)/composite \