hurd: Fix missing pthread_ compat symbol in libc

5476f8cd ("htl: move pthread_self info libc.") and
9dfa2562 ("htl: move pthread_equal into libc") to
1dc0bc8f ("htl: move pthread_attr_setdetachstate into libc")
moved some pthread_ symbols from libpthread.so to libc.so, but missed
adding the compat version like 5476f8cd ("htl: move pthread_self
info libc.") did: libc already had these symbols as forwards,
but versioned GLIBC_2.21, while the symbols in libpthread.so were
versioned GLIBC_2.12.

To fix running executables built before this, we thus have to add the
GLIBC_2.12 version, otherwise execution fails with e.g.

/usr/lib/i386-gnu/libglib-2.0.so: symbol lookup error: /usr/lib/i386-gnu/libglib-2.0.so: undefined symbol: pthread_attr_setinheritsched, version GLIBC_2.12