Remove #undef MBEDTLS_NET_C from mbedtls_config (#1128)

This was needed for windows because the `<windows.h>` include in
`threading_alt.h` broke compilation for `net_sockets.c`.

However, we can instead define `WIN32_LEAN_AND_MEAN` before including
<windows.h> in `threading_alt.h`, which avoids the issue so we can
remove this #undef.

src/hx/libs/ssl/mbedtls_config.h

@@ -5,6 +5,4 @@
 #define MBEDTLS_THREADING_PTHREAD
 #endif
 
-#undef MBEDTLS_NET_C
-
-#define MBEDTLS_THREADING_C
+#define MBEDTLS_THREADING_C

src/hx/libs/ssl/threading_alt.h

@@ -1,6 +1,7 @@
+#define WIN32_LEAN_AND_MEAN
 #include <windows.h>
 
 typedef struct {
 	CRITICAL_SECTION cs;
 	char is_valid;
-} mbedtls_threading_mutex_t;
+} mbedtls_threading_mutex_t;