Fix build with GCC 15
https://bugs.gentoo.org/944760

--- a/plain.src
+++ b/plain.src
@@ -112,7 +112,6 @@ static int  NSpace = 0;			    /* No. of spaces not flushed.   */
 
 /*---( forward )------------------------------------------------------------*/
 
-char *getenv();
 
 
 /*---( housekeeping )-------------------------------------------------------*/