windows: rewrite headers in lower case

Otherwise, compilation fails on systems that are case sensitive.

Reference: #159.

src/resolver.c

@@ -565,7 +565,7 @@ static int resolver_ares_srv_lookup(xmpp_ctx_t *ctx,
 #include <winsock2.h>
 #include <ws2tcpip.h>
 #include <windns.h>
-#include <Iphlpapi.h>
+#include <iphlpapi.h>
 
 struct dnsquery_header {
     unsigned short id;

src/sock.c

@@ -21,8 +21,8 @@
 #ifdef _WIN32
 #include <winsock2.h>
 #include <ws2tcpip.h>
-#include <Iphlpapi.h>
-#include <Mstcpip.h> /* tcp_keepalive */
+#include <iphlpapi.h>
+#include <mstcpip.h> /* tcp_keepalive */
 #else
 #include <errno.h>
 #include <unistd.h>