Clarify the intention of the include guard

This was a leftover of 27d5af59a3.

mjit_worker.c

@@ -1046,7 +1046,7 @@ compile_prelude(FILE *f)
     const char *s = pch_file;
     const char *e = header_name_end(s);
 
-# ifndef _MSC_VER // Visual Studio doesn't expect macro changes around headers. Anyway we don't support compaction there...
+# if USE_JIT_COMPACTION
     fprintf(f, "#ifndef MJIT_PCH\n");
     fprintf(f, "#define MJIT_PCH\n");
 # endif
@@ -1060,7 +1060,7 @@ compile_prelude(FILE *f)
         fputc(*s, f);
     }
     fprintf(f, "\"\n");
-# ifndef _MSC_VER
+# if USE_JIT_COMPACTION
     fprintf(f, "#endif\n");
 # endif
 #endif