build: fix dtrace-enabled build on os x

Commit 691d55f 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