Add <cstring> include to FastVector.h

This resolves the call to `std::memcpy` which was failing in recent skia
skia rolls with an error that looked like this:

    error: no member named 'memcpy' in namespace 'std'; did you mean simply 'memcpy'?

Bug: None
Change-Id: If54ed266812315cfea35d95a57c758f75bcb270b
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/4851062
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Auto-Submit: Arman Uguray <armansito@chromium.org>
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>

src/common/FastVector.h

@@ -16,6 +16,7 @@
 
 #include <algorithm>
 #include <array>
+#include <cstring>
 #include <initializer_list>
 #include <iterator>