build/solaris: drop duplicate __EXTENSION__ define

__EXTENSION__ is already defined in config.h by autoconf; drop the
duplicate from zebra.h.

Signed-off-by: David Lamparter <equinox@opensourcerouting.org>
diff --git a/lib/zebra.h b/lib/zebra.h
index 1ee5107..fe34be7 100644
--- a/lib/zebra.h
+++ b/lib/zebra.h
@@ -27,7 +27,6 @@
 
 #ifdef SUNOS_5
 #define _XPG4_2
-#define __EXTENSIONS__
 typedef unsigned int    u_int32_t;
 typedef unsigned short  u_int16_t;
 typedef unsigned char   u_int8_t;