(flags_argmatch): Return void, not int,

to pacify ./configure --enable-gcc-warnings.

src/getargs.c

@@ -81,7 +81,7 @@ extern char *program_name;
  *
  *  The special value 0 resets the flags to 0.
  */
-static int
+static void
 flags_argmatch (const char *option,
 		const char * const keys[], const int values[],
 		int *flags, char *args)