[Backtracing] Don't include things inside a namespace.

We shouldn't be including `<stdint.h>` inside the namespace.

stdlib/public/Backtracing/modules/FixedLayout.h

@@ -18,14 +18,14 @@
 #ifndef SWIFT_BACKTRACING_FIXED_LAYOUT_H
 #define SWIFT_BACKTRACING_FIXED_LAYOUT_H
 
+#include <stdint.h>
+
 #ifdef __cplusplus
 namespace swift {
 namespace runtime {
 namespace backtrace {
 #endif
 
-#include <stdint.h>
-
 struct x86_64_gprs {
   uint64_t _r[16];
   uint64_t rflags;