Different return type of backtrace function on FreeBSD

Jenkins is failing compilation on FreeBSD because its backtrace function is returning size_t instead of int. This leads to the compilation errors such as

../tor/src/common/backtrace.c:98:11: error: implicit conversion loses integer precision: 'size_t' (aka 'unsigned long') to 'int' [-Werror,-Wshorten-64-to-32]
  depth = backtrace(cb_buf, MAX_DEPTH);
        ~ ^~~~~~~~~~~~~~~~~~~~~~~~~~~~
../tor/src/common/backtrace.c:130:11: error: implicit conversion loses integer precision: 'size_t' (aka 'unsigned long') to 'int' [-Werror,-Wshorten-64-to-32]
  depth = backtrace(cb_buf, MAX_DEPTH);
        ~ ^~~~~~~~~~~~~~~~~~~~~~~~~~~~
../tor/src/common/backtrace.c:177:17: error: implicit conversion loses integer precision: 'size_t' (aka 'unsigned long') to 'int' [-Werror,-Wshorten-64-to-32]
    int depth = backtrace(cb_buf, MAX_DEPTH);
        ~~~~~   ^~~~~~~~~~~~~~~~~~~~~~~~~~~~