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