Add missing #include to fix build with GCC 10.

Bug: angleproject:4389
Change-Id: I4a2f04a12a059924871f5d54f137c67670747ebd
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2063042
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>

include/platform/Platform.h

@@ -10,6 +10,7 @@
 #define ANGLE_PLATFORM_H
 
 #include <stdint.h>
+#include <stdlib.h>
 #include <array>
 
 #define EGL_PLATFORM_ANGLE_PLATFORM_METHODS_ANGLEX 0x3482