Fix module-import-in-extern-c compiler error

This fixes a compiler error when building C++ projects that use the
'modules' C++ language feature consume Mbed TLS. The specific error is
module-import-in-extern-c, which occurs when `#include`s occur within
`extern C` blocks.

```
$ clang-17 -std=c++20 -fmodules -Itf-psa-crypto/include -Itf-psa-crypto/drivers/builtin/include -o sample sample.cpp
In file included from sample.cpp:8:
In file included from tf-psa-crypto/drivers/builtin/include/mbedtls/private/chachapoly.h:44:
tf-psa-crypto/drivers/builtin/include/mbedtls/private/chacha20.h:26:1: error: import of C++ module '_Builtin_stdint' appears within extern "C" language linkage specification [-Wmodule-import-in-extern-c]
   26 | #include <stdint.h>
      | ^
tf-psa-crypto/drivers/builtin/include/mbedtls/private/chachapoly.h:35:1: note: extern "C" language linkage specification begins here
   35 | extern "C" {
      | ^
1 error generated.
```

This seems to primarily be an issue with clang. See
https://clang.llvm.org/docs/ClangCommandLineReference.html#cmdoption-clang-fmodules

Signed-off-by: zacharykeyessonos <[email protected]>

Author: zacharykeyessonos

include/mbedtls/timing.h

@@ -15,10 +15,6 @@
 
 #include <stdint.h>
 
-#ifdef __cplusplus
-extern "C" {
-#endif
-
 #if !defined(MBEDTLS_TIMING_ALT)
 // Regular implementation
 //
@@ -43,6 +39,10 @@ typedef struct mbedtls_timing_delay_context {
 #include "timing_alt.h"
 #endif /* MBEDTLS_TIMING_ALT */
 
+#ifdef __cplusplus
+extern "C" {
+#endif
+
 /* Internal use */
 unsigned long mbedtls_timing_get_timer(struct mbedtls_timing_hr_time *val, int reset);