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.
3 jobs for wip/lantw/include-alloca-h-conditionally in 3 minutes and 18 seconds
Status Job ID Name Coverage
  Static Analysis
passed #7900
cppcheck

00:00:28

 
  Build
passed #7901
fedora_gcc

00:02:49

 
  Failure
skipped #7902
on_failure