build: include <cairo.h> not <cairo/cairo.h>

cairo.h is intended to be included as <cairo.h> not <cairo/cairo.h> as
is the style for clutter.h. If you have installed cairo to a custom
prefix then using cairo/cairo.h can result in unintentional use of the
system cairo headers, or if they aren't installed then it will result in
a failure to find the header.

clutter/clutter-stage.c

@@ -53,7 +53,7 @@
 #include "config.h"
 #endif
 
-#include <cairo/cairo.h>
+#include <cairo.h>
 
 #include "clutter-stage.h"
 

tests/conform/test-path.c

@@ -1,5 +1,5 @@
 #include <clutter/clutter.h>
-#include <cairo/cairo.h>
+#include <cairo.h>
 #include <string.h>
 #include <math.h>
 

tests/interactive/test-bin-layout.c

@@ -1,6 +1,6 @@
 #include <stdlib.h>
 #include <gmodule.h>
-#include <cairo/cairo.h>
+#include <cairo.h>
 #include <clutter/clutter.h>
 
 static const ClutterColor bg_color = { 0xcc, 0xcc, 0xcc, 0x99 };

tests/interactive/test-flow-layout.c

@@ -1,6 +1,6 @@
 #include <stdlib.h>
 #include <gmodule.h>
-#include <cairo/cairo.h>
+#include <cairo.h>
 #include <clutter/clutter.h>
 
 #define N_RECTS         20