profiler: Don't include alloca.h when disabled
alloca is a non-standard feature. It is possible for a system to put it in a different header, so including alloca.h unconditionally may cause compilation error. Since we only need alloca when this Linux-only profiler is enabled, we can simply move the include to the ifdef block.
wip/lantw/include-alloca-h-conditionally in 3 minutes and 18 seconds3 jobs for