acme: avoid global named "class"

For AIX.

src/cmd/acme/regx.c

@@ -15,6 +15,9 @@
 Rangeset	sel;
 Rune		*lastregexp;
 
+#undef class
+#define class regxclass /* some systems declare "class" in system headers */
+
 /*
  * Machine Information
  */