Fix missing-prototypes warnings

A missing include in helpers.c triggered a bunch of missing-prototypes
warnings.

BUG=None
TEST=Local build with -Wall is now free of warnings

Change-Id: I8d6655ba48abd5cb4f48fa29520f62b076a45690
Reviewed-on: https://chromium-review.googlesource.com/241441
Reviewed-by: Lauri Peltonen <lpeltonen@nvidia.com>
Commit-Queue: Lauri Peltonen <lpeltonen@nvidia.com>
Tested-by: Lauri Peltonen <lpeltonen@nvidia.com>
Reviewed-by: Stéphane Marchesin <marcheu@chromium.org>
diff --git a/helpers.c b/helpers.c
index 5d86451..caed6e6 100644
--- a/helpers.c
+++ b/helpers.c
@@ -10,6 +10,7 @@
 #include <xf86drm.h>
 
 #include "gbm_priv.h"
+#include "helpers.h"
 
 int gbm_bpp_from_format(uint32_t format)
 {