Add compile guard in platform.h

We may want to include it just for the errors it defines without having all of
the other defines that only make sense when PLATFORM_C is enabled.
diff --git a/include/mbedtls/platform.h b/include/mbedtls/platform.h
index 82d5e33..212c267 100644
--- a/include/mbedtls/platform.h
+++ b/include/mbedtls/platform.h
@@ -39,14 +39,16 @@
 #include MBEDTLS_CONFIG_FILE
 #endif
 
-#if defined(MBEDTLS_HAVE_TIME)
-#include "platform_time.h"
-#endif
-
 #define MBEDTLS_ERR_PLATFORM_HW_ACCEL_FAILED     -0x0070 /**< Hardware accelerator failed */
 #define MBEDTLS_ERR_PLATFORM_FEATURE_UNSUPPORTED -0x0072 /**< The requested feature is not supported by the platform */
 #define MBEDTLS_ERR_PLATFORM_FAULT_DETECTED      -0x0071 /**< A fault was detected in a critical path, likely indicative of an active physical attack */
 
+#if defined(MBEDTLS_PLATFORM_C)
+
+#if defined(MBEDTLS_HAVE_TIME)
+#include "platform_time.h"
+#endif
+
 #ifdef __cplusplus
 extern "C" {
 #endif
@@ -365,4 +367,6 @@
 }
 #endif
 
+#endif /* MBEDTLS_PLATFORM_C */
+
 #endif /* platform.h */