Fix build with GCC 15

https://bugs.gentoo.org/943982

--- a/src/c-lex.c
+++ b/src/c-lex.c
@@ -887,7 +887,7 @@ static inline int backslash(int c)
         case 'x':
             c1 = buffer_peek_char();
             if (is_hex_digit_char(c1)) {
-                return readx(c1);
+                return readx();
             } else {
                 Scm_Error("\\x must be followed by hex-digit, but got '%c'", c1);
             }
@@ -934,7 +934,7 @@ static inline int backslash(int c)
                         SCM_LIST1(SCM_MAKE_CHAR(c)));
                 return c;
             } else {
-                return buffer_read_char(c);
+                return buffer_read_char();
             }
         }
     } else {