.h files are only read when you recompile the code it applies to, they're not like running configuration files - so for the change to apply, you'd need to rebuild the FreeBSD kernel. Unsurprisingly, the FreeNAS distro doesn't include the source files or tools to do that.
You could do it on a normal system, then copy the kernel over - and hope you got all the other myriad configs right for a FreeNAS box.