tools/Makefile: FIT_CIPHER requires libssl

tools/Makefile: FIT_CIPHER requires libssl

If CONFIG_FIT_CIPHER is enabled without CONFIG_FIT_SIGNATURE then
mkimage/dumpimage will fail to link:

 /usr/bin/ld: tools/common/image-cipher.o: in function `fit_image_decrypt_data':
 image-cipher.c:(.text+0x9a): undefined reference to `image_get_host_blob'
 /usr/bin/ld: tools/common/image-cipher.o:(.data.rel+0x10): undefined reference to `EVP_aes_128_cbc'
 /usr/bin/ld: tools/common/image-cipher.o:(.data.rel+0x40): undefined reference to `EVP_aes_192_cbc'
 /usr/bin/ld: tools/common/image-cipher.o:(.data.rel+0x70): undefined reference to `EVP_aes_256_cbc'
 /usr/bin/ld: tools/lib/aes/aes-encrypt.o: in function `image_aes_encrypt':
 aes-encrypt.c:(.text+0x22): undefined reference to `EVP_CIPHER_CTX_new'
 /usr/bin/ld: aes-encrypt.c:(.text+0x6f): undefined reference to `EVP_EncryptInit_ex'
 /usr/bin/ld: aes-encrypt.c:(.text+0x8d): undefined reference to `EVP_EncryptUpdate'
 /usr/bin/ld: aes-encrypt.c:(.text+0xac): undefined reference to `EVP_CIPHER_CTX_free'
 /usr/bin/ld: aes-encrypt.c:(.text+0xf2): undefined reference to `EVP_EncryptFinal_ex'
 collect2: error: ld returned 1 exit status

Signed-off-by: Joel Stanley <joel@jms.id.au>