Ignore warnings for llvm15

Issue: #I6ID6E
Signed-off-by: Nguzhihao4 <guzhihao4@huawei.com>
Change-Id: I4452c47fd141b3f995f9fa212c8590671edd7e74

BUILD.gn

@@ -28,6 +28,7 @@ config("crypto_config") {
   cflags = [
     "-Wall",
     "-Wno-implicit-fallthrough",
+    "-Wno-unused-but-set-variable",
     "-pthread",
     "-fPIC",
     "-Wa,--noexecstack",