freebsd: abort usage in rpl_strerror_r

[Matthias Bolte]

Since e67e250d98e9dc0177a564f2cbfa99330902fe9e rpl_strerror_r contains
an abort call in a section of code that is compiled on FreeBSD. But
gcc complains about this

strerror_r.c: In function 'rpl_strerror_r':
strerror_r.c:503: warning: implicit declaration of function 'abort'
strerror_r.c:503: warning: incompatible implicit declaration of
built-in function 'abort'

Adding an unconditional include for stdlib.h fixes this problem. The
file already contains an include for stdlib.h under some condition, so
I'm not sure where in the condition tree to stick the include to fix
this problem properly.

Matthias