audio/deadbeef: fix build with clang 15

During an exp-run for llvm 15 (see bug 265425), it turned out that
audio/deadbeef failed to build with clang 15:

threading_pthread.c:57:12: error: incompatible pointer to integer conversion returning 'pthread_t' (aka 'struct pthread *') from a function with result type 'intptr_t' (aka 'long') [-Wint-conversion]
    return tid;
           ^~~

This is because it is trying to implicitly convert a pthread_t into an
integer. Add a cast to suppress the warning.

PR: 268991
Approved by: maintainer timeout (2 weeks)
MFH: 2023Q1