Add missing stdint.h include in span.h

Otherwise, the code does not compile with some versions of g++. In
Chromium, https://crrev.com/1132859 did the exact same thing.

Change-Id: I9617a233fa03a46518b2da41bbb56dcbffe2b829
Reviewed-on: https://pdfium-review.googlesource.com/c/pdfium/+/113430
Commit-Queue: Tom Sepez <tsepez@chromium.org>
Reviewed-by: Tom Sepez <tsepez@chromium.org>
Auto-Submit: Lei Zhang <thestig@chromium.org>
diff --git a/third_party/base/containers/span.h b/third_party/base/containers/span.h
index 00a2324..d11a393 100644
--- a/third_party/base/containers/span.h
+++ b/third_party/base/containers/span.h
@@ -6,6 +6,7 @@
 #define THIRD_PARTY_BASE_CONTAINERS_SPAN_H_
 
 #include <stddef.h>
+#include <stdint.h>
 
 #include <algorithm>
 #include <array>