timing_mbed.cpp: Check MBEDTLS_TIMING_ALT

Do not compile the Mbed implementation of Mbed TLS unless
MBEDTLS_TIMING_ALT is defined. This prevents a macro check error on
devices that do not have LPTICKER or USTICKER when Mbed TLS timing
is not enabled.

connectivity/mbedtls/platform/src/timing_mbed.cpp

@@ -24,6 +24,9 @@
 #else
 #include MBEDTLS_CONFIG_FILE
 #endif
+
+#if defined(MBEDTLS_TIMING_ALT)
+
 #include "mbedtls/timing.h"
 #include "drivers/Timeout.h"
 #include "drivers/Timer.h"
@@ -130,3 +133,5 @@ extern "C" int mbedtls_timing_get_delay(void *data)
 
     return 0;
 }
+
+#endif // MBEDTLS_TIMING_ALT