mark nimprof as used when profiling

lib/pure/nimprof.nim

@@ -14,6 +14,9 @@
 when not defined(profiler) and not defined(memProfiler):
   {.error: "Profiling support is turned off! Enable profiling by passing `--profiler:on --stackTrace:on` to the compiler (see the Nim Compiler User Guide for more options).".}
 
+when defined(nimHasUsed):
+  {.used.}
+
 # We don't want to profile the profiling code ...
 {.push profiler: off.}