include/u.h: fix name clash of REG_R* on solaris

Fixes build for solaris.

Change-Id: Ic6ffab36df9bd68fc38b258f1484a29fa2a0cd39
Reviewed-on: https://go-review.googlesource.com/4180Reviewed-by: Aram Hăvărneanu <aram@mgk.ro>

include/u.h

@@ -42,7 +42,7 @@ extern "C" {
 #define _NETBSD_SOURCE 1	/* NetBSD */
 #define _DEFAULT_SOURCE 1	/* glibc > 2.19 */
 #define _SVID_SOURCE 1
-#if !defined(__APPLE__) && !defined(__OpenBSD__)
+#if !defined(__APPLE__) && !defined(__OpenBSD__) && !defined(__sun__)
 #	define _XOPEN_SOURCE 1000
 #	define _XOPEN_SOURCE_EXTENDED 1
 #endif