fixed enet building on mingw

Makefile.am

@@ -1,5 +1,5 @@
 lib_LIBRARIES = libenet.a
 libenet_a_SOURCES = host.c list.c callbacks.c packet.c peer.c protocol.c unix.c win32.c
-INCLUDES = -Iinclude/
+INCLUDES = -Iinclude
 
 SUBDIRS = include