[builtins] Fix build after D95537 due to missing size_t

Strangely int_lib.h only includes stdint.h and not stddef.h, so we need
to include the latter to be able to use size_t here.

GitOrigin-RevId: e47a388bd6486d0af7066f1d8b033a13d0ffc7df
diff --git a/lib/builtins/gcc_personality_v0.c b/lib/builtins/gcc_personality_v0.c
index 9fb1ce0..cecbce1 100644
--- a/lib/builtins/gcc_personality_v0.c
+++ b/lib/builtins/gcc_personality_v0.c
@@ -7,6 +7,7 @@
 //===----------------------------------------------------------------------===//
 
 #include "int_lib.h"
+#include <stddef.h>
 
 #include <unwind.h>
 #if defined(__arm__) && !defined(__ARM_DWARF_EH__) &&                          \