Lots of warnings if dropbear built without dbclient

[pedro0311]

Like this one:

In file included from ../options.h:24,
                 from src/headers/tomcrypt_dropbear.h:2,
                 from src/headers/tomcrypt_custom.h:13,
                 from src/headers/tomcrypt.h:22,
                 from src/misc/error_to_string.c:10:
../sysoptions.h:314:27: warning: "DROPBEAR_CLIENT" is not defined

[mkj]

Thanks