gecko: hide DTRACE option where it's broken

building for: FreeBSD 93i386-default-job-01 9.3-RELEASE-p38 FreeBSD 9.3-RELEASE-p38 i386
Host OSVERSION: 1100097
Jail OSVERSION: 903000
[...]
dtrace -x nolibs -G -C -s .../js/src/devtools/javascript-trace.d -o js-dtrace.o RegExp.o ...
gmake[5]: *** [js-dtrace.o] Segmentation fault: 11 (core dumped)

Reported by: pkg-fallout