Adjust function definition in if_ovpn_c.c to avoid clang 15 warning

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

    tests/sys/net/if_ovpn/if_ovpn_c.c:19:14: error: a function declaration without a prototype is deprecated in all versions of C [-Werror,-Wstrict-prototypes]
    fake_sockaddr()
		 ^
		  void

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

MFC after: 3 days