[build] Suppress -Wunused-but-set-parameter/variable

This patch suppresses -Wunused-but-set-parameter and
-Wunused-but-set-variable warnings that goldfish code
does not comply with.
The code that causes the issues is as the following:
1) third_party/android/device/generic/goldfish-opengl/system/vulkan_enc/ResourceTracker.cpp:5191:33:
error: parameter 'input_result' set but not used
2) third_party/android/device/generic/goldfish-opengl/system/OpenglSystemCommon/ProcessPipe.cpp:223:10:
error: variable 'isPipe' set but not used
3) third_party/android/device/generic/goldfish-opengl/system/vulkan_enc/goldfish_vk_marshaling_guest.cpp:3270:51:
error: variable 'check_pInputAssemblyState' set but not used
4) third_party/android/device/generic/goldfish-opengl/system/vulkan_enc/goldfish_vk_marshaling_guest.cpp:3316:51:
error: variable 'check_pRasterizationState' set but not used
5) third_party/android/device/generic/goldfish-opengl/system/vulkan_enc/goldfish_vk_marshaling_guest.cpp:3260:49:
error: variable 'check_pVertexInputState' set but not used)

Change-Id: Icfec328a1dc239b046a1650e10aebc5a54be78c9
diff --git a/BUILD.gn b/BUILD.gn
index 2e204a7..f372452 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -134,6 +134,8 @@
     "-Wno-unused-value",
     "-Wno-unused-variable",
     "-Wno-conversion",
+    "-Wno-unused-but-set-parameter",
+    "-Wno-unused-but-set-variable",
   ]
 
   ldflags = [ "-static-libstdc++" ]