tools: fix ldgen_test fragment file KEEP

tools/test_apps/build_system/ldgen_test/main/linker.lf

@@ -4,5 +4,5 @@ entries:
     * (noflash)
     src1 (default)
     src1:func1 (noflash);
-        text->iram0_text KEEP ALIGN(9) ALIGN(12, post) SURROUND(sym1)
+        text->iram0_text KEEP() ALIGN(9) ALIGN(12, post) SURROUND(sym1)
     src1:func2 (rtc)