Suppress redefinition warnings of GET_SELF()

mjit_compile.c

@@ -348,6 +348,7 @@ mjit_compile_body(FILE *f, const rb_iseq_t *iseq, struct compile_status *status)
     fprintf(f, "    static const VALUE *const original_body_iseq = (VALUE *)0x%"PRIxVALUE";\n",
             (VALUE)body->iseq_encoded);
     fprintf(f, "    VALUE cfp_self = reg_cfp->self;\n"); // cache self across the method
+    fprintf(f, "#undef GET_SELF\n");
     fprintf(f, "#define GET_SELF() cfp_self\n");
 
     // Generate merged ivar guards first if needed