i915: add missing extensions.h include

Otherwise we'll bail with due to -Werror=implicit-function-declaration.
It went unnoticed since the we had a bug which did consistently set the
compiler flag.

Fixes: ba8a347f93 ("mesa: split extensions overrides and glGetString(GL_EXTENSIONS)")
Signed-off-by: Emil Velikov <emil.velikov@collabora.com>
Reviewed-by: Andres Gomez <agomez@igalia.com>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>

src/mesa/drivers/dri/i915/i830_context.c

@@ -27,6 +27,7 @@
 
 #include "i830_context.h"
 #include "main/api_exec.h"
+#include "main/extensions.h"
 #include "main/imports.h"
 #include "main/version.h"
 #include "main/vtxfmt.h"

src/mesa/drivers/dri/i915/i915_context.c

@@ -28,6 +28,7 @@
 #include "i915_context.h"
 #include "main/api_exec.h"
 #include "main/framebuffer.h"
+#include "main/extensions.h"
 #include "main/imports.h"
 #include "main/macros.h"
 #include "main/version.h"