strdup not available in MSVC compiler

- Added define to avoid the following warning, treated as an error
  by the nginx build system:
  warning C4996: 'strdup': The POSIX name for this item is deprecated. Instead, use the ISO C++ conformant name: _strdup. See online help for details.

src/ngx_http_modsecurity_module.c

@@ -25,6 +25,10 @@
 #include <ngx_core.h>
 #include <ngx_http.h>
 
+#ifdef WIN32
+#define strdup _strdup
+#endif
+
 static ngx_int_t ngx_http_modsecurity_init(ngx_conf_t *cf);
 static void *ngx_http_modsecurity_create_main_conf(ngx_conf_t *cf);
 static char *ngx_http_modsecurity_init_main_conf(ngx_conf_t *cf, void *conf);