Fix compilation issue on older mac (< 10.12)
Fix compilation issue on older mac (< 10.12)

Error was:
In file included from src/lib/entropy/getentropy/getentropy.cpp:13:
/usr/include/sys/random.h:37:32: error: unknown type name 'u_int'
void read_random(void* buffer, u_int numBytes);
                               ^
/usr/include/sys/random.h:38:33: error: unknown type name 'u_int'
void read_frandom(void* buffer, u_int numBytes);
                                ^
/usr/include/sys/random.h:39:33: error: unknown type name 'u_int'
int  write_random(void* buffer, u_int numBytes);
                                ^

randombit/botan,randombit/botan,randombit/botan,randombit/botan,randombit/botan