Make private tests symbols static

When testing the civetweb private APIs we need to make all the public
symbols static so that they do not conflict with the imported shared
library symbols.

test/private.c

@@ -21,9 +21,12 @@
  */
 
 /**
- * We include the source file so that we access to the internal private
+ * We include the source file so that we have access to the internal private
  * static functions
  */
+#ifdef _MSC_VER
+#define CIVETWEB_API static
+#endif
 #include "../src/civetweb.c"
 
 #include <stdlib.h>