Fix broken prototype.

Upstream bug: https://savannah.gnu.org/bugs/?66845

--- enscript-1.6.6/compat/regex.c	2011-10-30 16:48:42.000000000 +0000
+++ enscript-1.6.6/compat/regex.c	2025-07-03 17:19:20.821548319 +0100
@@ -336,7 +336,7 @@
 #define false 0
 #define true 1
 
-static int re_match_2_internal ();
+static int re_match_2_internal (struct re_pattern_buffer *, const char *, int, const char *, int, int, struct re_registers *, int);
 
 /* These are the command codes that appear in compiled regular
    expressions.  Some opcodes are followed by argument bytes.  A