build: prevent installation of client API example

This was never intended to be installed; waf just picked it up
automagically. There's also a closed ticket on github where someone
complains that the program "simple" is installed, and I didn't realize
at this point that it was actually installed by default when enabling
the client API.

wscript_build.py

@@ -486,6 +486,7 @@ def build(ctx):
                 includes     = [ctx.bldnode.abspath(), ctx.srcnode.abspath()],
                 use          = "mpv",
                 features     = "c cprogram",
+                install_path = None
             )
 
     if ctx.env.DEST_OS == 'win32':