[CentOS] compiling Zaptel-1.2.4 on CentOS 4.3

[William Suffill]

Seems there is bug in the kernel-devel package that can cause some
compile problems also on Centos is seems to work better if u symlink
/usr/src/linux to /usr/src/kernels/`uname -r`.

https://bugzilla.redhat.com/bugzilla/show_bug.cgi?id=180568. There is
also a Digium bug about spinlock too.

http://bugs.digium.com/view.php?id=6425

The fix is just to fix a small typo in spinlock.h

-- Wlliam