Adjust parse() definition in boot2 to avoid clang 15 warning

With clang 15, the following -Werror warning is produced:

    stand/i386/boot2/boot2.c:358:6: error: a function declaration without a prototype is deprecated in all versions of C [-Werror,-Wstrict-prototypes]

This is because parse() is declared with a (void) argument list, and
defined with an empty argument list. Make the definition match the
declaration.

MFC after:	3 days

stand/i386/boot2/boot2.c

@@ -355,7 +355,7 @@ load(void)
 }
 
 static int
-parse()
+parse(void)
 {
 	char *arg, *ep, *p, *q;
 	const char *cp;