libweston: add missing include

Fixes missing prototypes compilation warnings emitted when a function
is defined before its prototype is declared.

These warnings were introduced over time since the switch to meson
because the -Wmissing-protoypes was not included in the compilation
arguments.

Signed-off-by: Guillaume Champagne <champagne.guillaume.c@gmail.com>
diff --git a/libweston/data-device.c b/libweston/data-device.c
index 865d749..8b0a282 100644
--- a/libweston/data-device.c
+++ b/libweston/data-device.c
@@ -33,6 +33,7 @@
 #include <assert.h>
 
 #include <libweston/libweston.h>
+#include "libweston-internal.h"
 #include "shared/helpers.h"
 #include "shared/timespec-util.h"