tools/fit_check_sign: make the module dependent on CONFIG_FIT_SIGNATURE

tools/fit_check_sign: make the module dependent on CONFIG_FIT_SIGNATURE

The function definition of fit_check_sign() is guarded by
"#ifdef CONFIG_FIT_SIGNATURE" in "tools/image-host.c". If we try
to build it without CONFIG_FIT_SIGNATURE, we will get an error:

/usr/bin/ld: tools/fit_check_sign.o: in function `main':
fit_check_sign.c:(.text.startup+0x165): undefined reference to `fit_check_sign'
collect2: error: ld returned 1 exit status

Fixes: 9c79c8fe70da ("tools/fit_check_sign: make key optional")
Signed-off-by: Shiji Yang <yangshiji66@outlook.com>