[ExecuTorch] Fix missing cstdint in vec_base.h (#6107)

Fix missing cstdint in vec_base.h (#5747)

Summary:
Pull Request resolved: #5747

This header uses int32_t etc and doesn't have this include.
ghstack-source-id: 245437209
exported-using-ghexport

Reviewed By: kimishpatel

Differential Revision: D63638423

fbshipit-source-id: 594c3e8de20699e964e54cde732968c237bd0a72
(cherry picked from commit e2f1aca)

Co-authored-by: Scott Wolchok <[email protected]>

Author: pytorchbot

kernels/optimized/vec/vec_base.h

@@ -3,6 +3,7 @@
 // @nolint PATTERNLINT <functional> is required for std::equal_to, etc.
 
 #include <cassert>
+#include <cstdint>
 #include <cstring>
 #include <functional>
 #include <cmath>