meson: dep fix for compositor.h needing xkbcommon.h

This fixes:

[ 5s] cc -Ilibweston/2b98b6d@@session-helper@sta -Ilibweston -I../libweston -Ilibweston/.. -I../libweston/.. -Ilibwes ton/../shared -I../libweston/../shared -I/usr/include/pixman-1 -I/usr/include/libdrm -I/usr/include/dbus-1.0 -I/usr/lib6 4/dbus-1.0/include -fdiagnostics-color=always -pipe -D_FILE_OFFSET_BITS=64 -std=gnu99 -Wno-unused-parameter -Wno-shift-n egative-value -Wno-missing-field-initializers -fvisibility=hidden -O2 -Wall -D_FORTIFY_SOURCE=2 -fstack-protector-strong -funwind-tables -fasynchronous-unwind-tables -fstack-clash-protection -g -fPIC -MD -MQ 'libweston/2b98b6d@@session-hel per@sta/launcher-util.c.o' -MF 'libweston/2b98b6d@@session-helper@sta/launcher-util.c.o.d' -o 'libweston/2b98b6d@@sessio n-helper@sta/launcher-util.c.o' -c ../libweston/launcher-util.c [ 5s] In file included from ../libweston/launcher-util.c:29: [ 5s] ../libweston/compositor.h:39:10: fatal error: xkbcommon/xkbcommon.h: No such file or directory [ 5s] #include <xkbcommon/xkbcommon.h>

Reported in https://lists.freedesktop.org/archives/wayland-devel/2019-March/040358.html