shared/platform.h: add missing stdbool.h include

Required by the bool type, used through the header.

Signed-off-by: Emil Velikov <emil.velikov@collabora.com>
Reviewed-by: Daniel Stone <daniels@collabora.com>
diff --git a/shared/platform.h b/shared/platform.h
index dd55008..b1b9128 100644
--- a/shared/platform.h
+++ b/shared/platform.h
@@ -26,6 +26,7 @@
 #ifndef WESTON_PLATFORM_H
 #define WESTON_PLATFORM_H
 
+#include <stdbool.h>
 #include <string.h>
 
 #ifdef ENABLE_EGL