IWYU: add string.h for strncmp in CFX_GifContext

Bug: chromium:957519
Change-Id: Ib650e567c1716f818af3aded388acb7c7dd33bcb
Reviewed-on: https://pdfium-review.googlesource.com/c/pdfium/+/110290
Reviewed-by: Lei Zhang <thestig@chromium.org>
Commit-Queue: Lei Zhang <thestig@chromium.org>
diff --git a/core/fxcodec/gif/cfx_gifcontext.cpp b/core/fxcodec/gif/cfx_gifcontext.cpp
index e7fc432..9811759 100644
--- a/core/fxcodec/gif/cfx_gifcontext.cpp
+++ b/core/fxcodec/gif/cfx_gifcontext.cpp
@@ -7,6 +7,7 @@
 #include "core/fxcodec/gif/cfx_gifcontext.h"
 
 #include <stdint.h>
+#include <string.h>
 
 #include <algorithm>
 #include <iterator>