eg-sampler: Fix default gain value

The initial gain coefficient in the code is 1.0 (full volume).  The parameter
itself is in dB, though, so the matching external default value is zero.

plugins/eg-sampler.lv2/sampler.ttl

@@ -58,7 +58,7 @@
 	] ;
 	state:state [
 		<http://lv2plug.in/plugins/eg-sampler#sample> <click.wav> ;
-		param:gain "1.0"^^xsd:float
+		param:gain "0.0"^^xsd:float
 	] .
 
 <http://lv2plug.in/plugins/eg-sampler#ui>