I was able to get the ladspa.h file
by downloading ladspa-sdk (1.13-1) from
http://packages.debian.org/squeeze/ladspa-sdk

I opened up the debian package and grabbed ladspa.h
in /usr/include/

I found the following instructions in the sources:

The ladspa.h API header file is needed to build hosts and plugins.
It may be useful to
install it in /usr/local/include/ or
/usr/include/. The programs generated in the
bin/ directory may be moved to
/usr/local/bin/ or /usr/bin/ and the plugins
generated in the plugins/ directory may be moved to
/usr/local/lib/ladspa/ or
/usr/lib/ladspa/

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