added missed ifdef HAVE_DLOPEN flag

The compilation of OTP for embedded systems (e.g. Android, Raspberry PI) with -static flags is failed;

obj/arm-unknown-linux-androideabi/opt/plain/erl_unix_sys_ddll.o:erl_unix_sys_ddll.c:function erts_sys_ddll_open_noext: error: undefined reference to 'dlerror'
obj/arm-unknown-linux-androideabi/opt/plain/erl_unix_sys_ddll.o:erl_unix_sys_ddll.c:function erts_sys_ddll_open_noext: error: undefined reference to 'dlopen'
obj/arm-unknown-linux-androideabi/opt/plain/erl_unix_sys_ddll.o:erl_unix_sys_ddll.c:function erts_sys_ddll_open_noext: error: undefined reference to 'dlerror'

The function erts_sys_ddll_open_noext(...) is missing #if defined(HAVE_DLOPEN) ... #else ... #endif constrain