Fixes an undef compilation warning

[fisiognomico]

You may have noticed lately a warning during compilation of Julia master introduced by #27466

In file included from /home/rodia/dev/julia/src/jitlayers.h:19,
                 from /home/rodia/dev/julia/src/codegen.cpp:107:
/home/rodia/dev/julia/usr/include/llvm/ExecutionEngine/JITEventListener.h:118:5: warning: 
"LLVM_USE_PERF" is not defined, evaluates to 0 [-Wundef]
 #if LLVM_USE_PERF
     ^~~~~~~~~~~~~
    CC src/disasm.o
In file included from /home/rodia/dev/julia/src/jitlayers.h:19,
                 from /home/rodia/dev/julia/src/jitlayers.cpp:69:
/home/rodia/dev/julia/usr/include/llvm/ExecutionEngine/JITEventListener.h:118:5: warning: 
"LLVM_USE_PERF" is not defined, evaluates to 0 [-Wundef]
 #if LLVM_USE_PERF
     ^~~~~~~~~~~~~
In file included from /home/rodia/dev/julia/src/disasm.cpp:62:
/home/rodia/dev/julia/usr/include/llvm/ExecutionEngine/JITEventListener.h:118:5: warning: 
"LLVM_USE_PERF" is not defined, evaluates to 0 [-Wundef]
 #if LLVM_USE_PERF
     ^~~~~~~~~~~~~

This PR fixes it.

[fisiognomico]

One of the three Travis tests errored due to a machine shutdown during the test.