psplash-fb: fix sign-compare warning

Cast font->index[i] to wchar_t for the comparison.

This fixes following sign-compare warning:

psplash-fb.c: In function 'psplash_font_glyph':
psplash-fb.c:452:33: warning: comparison between signed and unsigned
integer expressions [-Wsign-compare]

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