build: fix dtrace-enabled build on os x

Commit 691d55fe introduces a regression on OS X when dtrace is enabled (the default.) This commit rectifies that by removing the erroneous platform check.

Fixes the following build error:

CXX(target) /Users/bnoordhuis/src/iojs/out/Release/obj.target/node/src/node_dtrace.o
../src/node_dtrace.cc:27:10: fatal error: 'node_provider.h' file not found

R=@bajtos, /cc @evantorrie