Fix build from cmake

George reports:

I spent a few hours on this. Might save someone else some time.

I'm working in a larger project that uses cmake for its builds. cmake
supports adding external projects using the ExternalProject_Add
function.

But, there is some issue in the fio Makefile that causes a problem
when run from cmake. I could never reproduce it from the command line.

I was able to get the build to happen by changing the implicit rule
.c.o to use pattern notation. Not sure if this will work in all cases
or has other side effects.

Without this change, I was getting this error.

make[4]: *** No rule to make target `gettime.o', needed by `fio'.  Stop.

Signed-off-by: Jens Axboe <axboe@fb.com>