Fix _P use on OpenBSD

git-svn-id: svn://svn.savannah.nongnu.org/qemu/trunk@6298 c046a42c-6fe2-441c-8c8c-71466251a162

slirp/slirp.h

@@ -120,13 +120,12 @@ typedef unsigned char u_int8_t;
 #include <sys/uio.h>
 #endif
 
-#ifndef _P
+#undef _P
 #ifndef NO_PROTOTYPES
 #  define   _P(x)   x
 #else
 #  define   _P(x)   ()
 #endif
-#endif
 
 #ifndef _WIN32
 #include <netinet/in.h>