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/log.c b/libweston/log.c
index 6ce9454..da57e03 100644
--- a/libweston/log.c
+++ b/libweston/log.c
@@ -35,6 +35,7 @@
 #include <wayland-util.h>
 
 #include <libweston/libweston.h>
+#include "weston-log-internal.h"
 
 /**
  * \defgroup wlog weston-logging