dropbear: fix compilation for uClibc

dropbear: fix compilation for uClibc

Backport patches which fix compile issue for uClibc-ng :

dbrandom.c:174:8: warning: implicit declaration of function 'getrandom'; did you mean 'genrandom'? [-Wimplicit-function-declaration]
  ret = getrandom(buf, sizeof(buf), GRND_NONBLOCK);
        ^~~~~~~~~
        genrandom
dbrandom.c:174:36: error: 'GRND_NONBLOCK' undeclared (first use in this function); did you mean 'SOCK_NONBLOCK'?
  ret = getrandom(buf, sizeof(buf), GRND_NONBLOCK);
                                    ^~~~~~~~~~~~~
                                    SOCK_NONBLOCK

Signed-off-by: Hans Dedecker <dedeckeh@gmail.com>