Fix yyerror ftbfs under Fedora 40

Signed-off-by: François Andriot <albator78@libertysurf.fr>

sipgen/lexer.l

@@ -21,6 +21,7 @@
 #include <stdlib.h>
 #include <string.h>
 #include <errno.h>
+#include <unistd.h>
 
 #include "sip.h"
 #include "parser.h"

sipgen/parser.y

@@ -23,6 +23,7 @@
 
 #include "sip.h"
 
+void yyerror(char* s);
 
 #define MAX_NESTED_IF       10
 #define MAX_NESTED_SCOPE    10