[build] Suppress -Wunused-but-set-variable in quickjs

This patch suppresses -Wunused-but-set-variable warning in
quickjs third party code that does not comply with.

The code that causes the issues is as the following:
1) third_party/quickjs/quickjs.c:7353:10: error: variable
'atom1_is_integer' set but not used
2) third_party/quickjs/quickjs.c:7353:28: error: variable
'atom2_is_integer' set but not used
3) third_party/quickjs/quickjs.c:45390:17: error: variable 's' set but
not used

Bug: 77973
Change-Id: Ie6f9fe06953caa4f0280517d04cc2b46a41d4afb
diff --git a/BUILD.gn b/BUILD.gn
index 2efef67..85353d9 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -27,6 +27,7 @@
     "-Wno-implicit-fallthrough",
     "-Wno-implicit-int-float-conversion",
     "-Wno-sign-compare",
+    "-Wno-unused-but-set-variable",
     "-Wno-unused-variable",
     "-funsigned-char",
   ]