Declare mbedtls_debug_print_msg as printf-like

We were not getting any warnings on printf format errors, as we do not
explicitly use printf anywhere in the code. Thankfully there is a way
to mark a function as having printf behaviour so that its inputs can be
checked in the same way as printf would be.

Signed-off-by: Paul Elliott <paul.elliott@arm.com>

library/debug.c

@@ -74,6 +74,7 @@ static inline void debug_send_line( const mbedtls_ssl_context *ssl, int level,
 #endif
 }
 
+MBEDTLS_PRINTF_ATTRIBUTE(5, 6)
 void mbedtls_debug_print_msg( const mbedtls_ssl_context *ssl, int level,
                               const char *file, int line,
                               const char *format, ... )