psplash: fix remaining unused-parameter warnings

Define UNUSED macro for GCC's ((__unused__)) attribute and use it for
the remaining unused-parameter warnings. These unused parameters are
required due to the definition of the sighandler_t.

This fixes following unused-parameter warnings:

psplash.c: In function 'psplash_exit':
psplash.c:36:19: warning: unused parameter 'signum' [-Wunused-parameter]

psplash-console.c: In function 'vt_request':
psplash-console.c:27:17: warning: unused parameter 'sig'
[-Wunused-parameter]

Signed-off-by: Richard Leitner <richard.leitner@skidata.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>