psplash-fb: remove unused parameter from psplash_fb_text_size

This fixes following unused-parameter warning:

psplash-fb.c: In function 'psplash_fb_text_size':
psplash-fb.c:464:43: warning: unused parameter 'fb' [-Wunused-parameter]

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