mark toEmbree converters static

Trying to compile from e3b197feeb06ff8ebfd9a31415a00a4cdeb9a64b on Xcode 4.6.3, I get a linker error `ld: 8 duplicate symbols for architecture i386`, with the duplicate symbols being the toEmbree() converters in ofxEmbree.h. 

This PR just marks them static, which compiles / runs successfully.