build: add missing dep to x11 backend

All other backends already link to libweston, x11 backend should too.

This fixes a build failure:

[1/50] Compiling C object 'libweston/2b98b6d@@x11-backend@sha/compositor-x11.c.o'.
FAILED: libweston/2b98b6d@@x11-backend@sha/compositor-x11.c.o
cc -Ilibweston/2b98b6d@@x11-backend@sha -Ilibweston -I../../git/weston/libweston -Ilibweston/.. -I../../git/weston/libweston/.. -Ilibweston/../shared -I../../git/weston/libweston/../shared -Iprotocol -I/home/pq/local/include -I/usr/include/cairo -I/usr/include/glib-2.0 -I/usr/lib/x86_64-linux-gnu/glib-2.0/include -I/usr/include/pixman-1 -I/usr/include/uuid -I/usr/include/freetype2 -I/usr/include/libpng16 -I/usr/include/pango-1.0 -I/usr/include/fribidi -I/usr/include/harfbuzz -I/usr/include/libdrm -fdiagnostics-color=always -pipe -D_FILE_OFFSET_BITS=64 -Wall -Winvalid-pch -Wextra -std=gnu99 -g -Wno-unused-parameter -Wno-shift-negative-value -Wno-missing-field-initializers -fvisibility=hidden -fPIC  -MD -MQ 'libweston/2b98b6d@@x11-backend@sha/compositor-x11.c.o' -MF 'libweston/2b98b6d@@x11-backend@sha/compositor-x11.c.o.d' -o 'libweston/2b98b6d@@x11-backend@sha/compositor-x11.c.o' -c ../../git/weston/libweston/compositor-x11.c
../../git/weston/libweston/compositor-x11.c:51:10: fatal error: xkbcommon/xkbcommon.h: No such file or directory
 #include <xkbcommon/xkbcommon.h>

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.com>

libweston/meson.build

@@ -365,6 +365,7 @@ if get_option('backend-x11')
 	endif
 
 	deps_x11 = [
+		dep_libweston,
 		dep_x11_xcb,
 		dep_lib_cairo_shared,
 		dep_pixman,