Include fixed snprintf for Windows in platform.c

Use _WIN32 to detect it rather that _MSC_VER as it turns out MSYS2 uses the
broken MS version by default too.
diff --git a/include/mbedtls/check_config.h b/include/mbedtls/check_config.h
index 9fb870a..2445031 100644
--- a/include/mbedtls/check_config.h
+++ b/include/mbedtls/check_config.h
@@ -39,6 +39,10 @@
 #error "mbed TLS requires a platform with 8-bit chars"
 #endif
 
+#if defined(_WIN32) && !defined(MBEDTLS_PLATFORM_C)
+#error "MBEDTLS_PLATFORM_C is required on Windows"
+#endif
+
 #if defined(MBEDTLS_DEPRECATED_WARNING) && \
     !defined(__GNUC__) && !defined(__clang__)
 #error "MBEDTLS_DEPRECATED_WARNING only works with GCC and Clang"